/* Storage object read/write
 *
 * Copyright (C) 2007 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells (dhowells@redhat.com)
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public Licence
 * as published by the Free Software Foundation; either version
 * 2 of the Licence, or (at your option) any later version.
 */

#include <linux/mount.h>
#include <linux/slab.h>
#include <linux/file.h>
#include "internal.h"

/*
 * detect wake up events generated by the unlocking of pages in which we're
 * interested
 * - we use this to detect read completion of backing pages
 * - the caller holds the waitqueue lock
 */
static int cachefiles_read_waiter(wait_queue_t *wait, unsigned mode,
                                  int sync, void *_key)
{
        struct cachefiles_one_read *monitor =
                container_of(wait, struct cachefiles_one_read, monitor);
        struct cachefiles_object *object;
        struct wait_bit_key *key = _key;
        struct page *page = wait->private;

        ASSERT(key);

        _enter("{%lu},%u,%d,{%p,%u}",
               monitor->netfs_page->index, mode, sync,
               key->flags, key->bit_nr);

        if (key->flags != &page->flags ||
            key->bit_nr != PG_locked)
                return 0;

        _debug("--- monitor %p %lx ---", page, page->flags);

        if (!PageUptodate(page) && !PageError(page)) {
                /* unlocked, not uptodate and not erronous? */
                _debug("page probably truncated");
        }

        /* remove from the waitqueue */
        list_del(&wait->task_list);

        /* move onto the action list and queue for FS-Cache thread pool */
        ASSERT(monitor->op);

        object = container_of(monitor->op->op.object,
                              struct cachefiles_object, fscache);

        spin_lock(&object->work_lock);
        list_add_tail(&monitor->op_link, &monitor->op->to_do);
        spin_unlock(&object->work_lock);

        fscache_enqueue_retrieval(monitor->op);
        return 0;
}

/*
 * handle a probably truncated page
 * - check to see if the page is still relevant and reissue the read if
 *   possible
 * - return -EIO on error, -ENODATA if the page is gone, -EINPROGRESS if we
 *   must wait again and 0 if successful
 */
static int cachefiles_read_reissue(struct cachefiles_object *object,
                                   struct cachefiles_one_read *monitor)
{
        struct address_space *bmapping = object->backer->d_inode->i_mapping;
        struct page *backpage = monitor->back_page, *backpage2;
        int ret;

        _enter("{ino=%lx},{%lx,%lx}",
               object->backer->d_inode->i_ino,
               backpage->index, backpage->flags);

        /* skip if the page was truncated away completely */
        if (backpage->mapping != bmapping) {
                _leave(" = -ENODATA [mapping]");
                return -ENODATA;
        }

        backpage2 = find_get_page(bmapping, backpage->index);
        if (!backpage2) {
                _leave(" = -ENODATA [gone]");
                return -ENODATA;
        }

        if (backpage != backpage2) {
                put_page(backpage2);
                _leave(" = -ENODATA [different]");
                return -ENODATA;
        }

        /* the page is still there and we already have a ref on it, so we don't
         * need a second */
        put_page(backpage2);

        INIT_LIST_HEAD(&monitor->op_link);
        add_page_wait_queue(backpage, &monitor->monitor);

        if (trylock_page(backpage)) {
                ret = -EIO;
                if (PageError(backpage))
                        goto unlock_discard;
                ret = 0;
                if (PageUptodate(backpage))
                        goto unlock_discard;

                _debug("reissue read");
                ret = bmapping->a_ops->readpage(NULL, backpage);
                if (ret < 0)
                        goto unlock_discard;
        }

        /* but the page may have been read before the monitor was installed, so
         * the monitor may miss the event - so we have to ensure that we do get
         * one in such a case */
        if (trylock_page(backpage)) {
                _debug("jumpstart %p {%lx}", backpage, backpage->flags);
                unlock_page(backpage);
        }

        /* it'll reappear on the todo list */
        _leave(" = -EINPROGRESS");
        return -EINPROGRESS;

unlock_discard:
        unlock_page(backpage);
        spin_lock_irq(&object->work_lock);
        list_del(&monitor->op_link);
        spin_unlock_irq(&object->work_lock);
        _leave(" = %d", ret);
        return ret;
}

/*
 * copy data from backing pages to netfs pages to complete a read operation
 * - driven by FS-Cache's thread pool
 */
static void cachefiles_read_copier(struct fscache_operation *_op)
{
        struct cachefiles_one_read *monitor;
        struct cachefiles_object *object;
        struct fscache_retrieval *op;
        struct pagevec pagevec;
        int error, max;

        op = container_of(_op, struct fscache_retrieval, op);
        object = container_of(op->op.object,
                              struct cachefiles_object, fscache);

        _enter("{ino=%lu}", object->backer->d_inode->i_ino);

        pagevec_init(&pagevec, 0);

        max = 8;
        spin_lock_irq(&object->work_lock);

        while (!list_empty(&op->to_do)) {
                monitor = list_entry(op->to_do.next,
                                     struct cachefiles_one_read, op_link);
                list_del(&monitor->op_link);

                spin_unlock_irq(&object->work_lock);

                _debug("- copy {%lu}", monitor->back_page->index);

        recheck:
                if (test_bit(FSCACHE_COOKIE_INVALIDATING,
                             &object->fscache.cookie->flags)) {
                        error = -ESTALE;
                } else if (PageUptodate(monitor->back_page)) {
                        copy_highpage(monitor->netfs_page, monitor->back_page);
                        fscache_mark_page_cached(monitor->op,
                                                 monitor->netfs_page);
                        error = 0;
                } else if (!PageError(monitor->back_page)) {
                        /* the page has probably been truncated */
                        error = cachefiles_read_reissue(object, monitor);
                        if (error == -EINPROGRESS)
                                goto next;
                        goto recheck;
                } else {
                        cachefiles_io_error_obj(
                                object,
                                "Readpage failed on backing file %lx",
                                (unsigned long) monitor->back_page->flags);
                        error = -EIO;
                }

                page_cache_release(monitor->back_page);

                fscache_end_io(op, monitor->netfs_page, error);
                page_cache_release(monitor->netfs_page);
                fscache_retrieval_complete(op, 1);
                fscache_put_retrieval(op);
                kfree(monitor);

        next:
                /* let the thread pool have some air occasionally */
                max--;
                if (max < 0 || need_resched()) {
                        if (!list_empty(&op->to_do))
                                fscache_enqueue_retrieval(op);
                        _leave(" [maxed out]");
                        return;
                }

                spin_lock_irq(&object->work_lock);
        }

        spin_unlock_irq(&object->work_lock);
        _leave("");
}

/*
 * read the corresponding page to the given set from the backing file
 * - an uncertain page is simply discarded, to be tried again another time
 */
static int cachefiles_read_backing_file_one(struct cachefiles_object *object,
                                            struct fscache_retrieval *op,
                                            struct page *netpage,
                                            struct pagevec *pagevec)
{
        struct cachefiles_one_read *monitor;
        struct address_space *bmapping;
        struct page *newpage, *backpage;
        int ret;

        _enter("");

        pagevec_reinit(pagevec);

        _debug("read back %p{%lu,%d}",
               netpage, netpage->index, page_count(netpage));

        monitor = kzalloc(sizeof(*monitor), cachefiles_gfp);
        if (!monitor)
                goto nomem;

        monitor->netfs_page = netpage;
        monitor->op = fscache_get_retrieval(op);

        init_waitqueue_func_entry(&monitor->monitor, cachefiles_read_waiter);

        /* attempt to get hold of the backing page */
        bmapping = object->backer->d_inode->i_mapping;
        newpage = NULL;

        for (;;) {
                backpage = find_get_page(bmapping, netpage->index);
                if (backpage)
                        goto backing_page_already_present;

                if (!newpage) {
                        newpage = __page_cache_alloc(cachefiles_gfp |
                                                     __GFP_COLD);
                        if (!newpage)
                                goto nomem_monitor;
                }

                ret = add_to_page_cache(newpage, bmapping,
                                        netpage->index, cachefiles_gfp);
                if (ret == 0)
                        goto installed_new_backing_page;
                if (ret != -EEXIST)
                        goto nomem_page;
        }

        /* we've installed a new backing page, so now we need to add it
         * to the LRU list and start it reading */
installed_new_backing_page:
        _debug("- new %p", newpage);

        backpage = newpage;
        newpage = NULL;

        page_cache_get(backpage);
        pagevec_add(pagevec, backpage);
        __pagevec_lru_add_file(pagevec);

read_backing_page:
        ret = bmapping->a_ops->readpage(NULL, backpage);
        if (ret < 0)
                goto read_error;

        /* set the monitor to transfer the data across */
monitor_backing_page:
        _debug("- monitor add");

        /* install the monitor */
        page_cache_get(monitor->netfs_page);
        page_cache_get(backpage);
        monitor->back_page = backpage;
        monitor->monitor.private = backpage;
        add_page_wait_queue(backpage, &monitor->monitor);
        monitor = NULL;

        /* but the page may have been read before the monitor was installed, so
         * the monitor may miss the event - so we have to ensure that we do get
         * one in such a case */
        if (trylock_page(backpage)) {
                _debug("jumpstart %p {%lx}", backpage, backpage->flags);
                unlock_page(backpage);
        }
        goto success;

        /* if the backing page is already present, it can be in one of
         * three states: read in progress, read failed or read okay */
backing_page_already_present:
        _debug("- present");

        if (newpage) {
                page_cache_release(newpage);
                newpage = NULL;
        }

        if (PageError(backpage))
                goto io_error;

        if (PageUptodate(backpage))
                goto backing_page_already_uptodate;

        if (!trylock_page(backpage))
                goto monitor_backing_page;
        _debug("read %p {%lx}", backpage, backpage->flags);
        goto read_backing_page;

        /* the backing page is already up to date, attach the netfs
         * page to the pagecache and LRU and copy the data across */
backing_page_already_uptodate:
        _debug("- uptodate");

        fscache_mark_page_cached(op, netpage);

        copy_highpage(netpage, backpage);
        fscache_end_io(op, netpage, 0);
        fscache_retrieval_complete(op, 1);

success:
        _debug("success");
        ret = 0;

out:
        if (backpage)
                page_cache_release(backpage);
        if (monitor) {
                fscache_put_retrieval(monitor->op);
                kfree(monitor);
        }
        _leave(" = %d", ret);
        return ret;

read_error:
        _debug("read error %d", ret);
        if (ret == -ENOMEM) {
                fscache_retrieval_complete(op, 1);
                goto out;
        }
io_error:
        cachefiles_io_error_obj(object, "Page read error on backing file");
        fscache_retrieval_complete(op, 1);
        ret = -ENOBUFS;
        goto out;

nomem_page:
        page_cache_release(newpage);
nomem_monitor:
        fscache_put_retrieval(monitor->op);
        kfree(monitor);
nomem:
        fscache_retrieval_complete(op, 1);
        _leave(" = -ENOMEM");
        return -ENOMEM;
}

/*
 * read a page from the cache or allocate a block in which to store it
 * - cache withdrawal is prevented by the caller
 * - returns -EINTR if interrupted
 * - returns -ENOMEM if ran out of memory
 * - returns -ENOBUFS if no buffers can be made available
 * - returns -ENOBUFS if page is beyond EOF
 * - if the page is backed by a block in the cache:
 *   - a read will be started which will call the callback on completion
 *   - 0 will be returned
 * - else if the page is unbacked:
 *   - the metadata will be retained
 *   - -ENODATA will be returned
 */
int cachefiles_read_or_alloc_page(struct fscache_retrieval *op,
                                  struct page *page,
                                  gfp_t gfp)
{
        struct cachefiles_object *object;
        struct cachefiles_cache *cache;
        struct pagevec pagevec;
        struct inode *inode;
        sector_t block0, block;
        unsigned shift;
        int ret;

        object = container_of(op->op.object,
                              struct cachefiles_object, fscache);
        cache = container_of(object->fscache.cache,
                             struct cachefiles_cache, cache);

        _enter("{%p},{%lx},,,", object, page->index);

        if (!object->backer)
                goto enobufs;

        inode = object->backer->d_inode;
        ASSERT(S_ISREG(inode->i_mode));
        ASSERT(inode->i_mapping->a_ops->bmap);
        ASSERT(inode->i_mapping->a_ops->readpages);

        /* calculate the shift required to use bmap */
        if (inode->i_sb->s_blocksize > PAGE_SIZE)
                goto enobufs;

        shift = PAGE_SHIFT - inode->i_sb->s_blocksize_bits;

        op->op.flags &= FSCACHE_OP_KEEP_FLAGS;
        op->op.flags |= FSCACHE_OP_ASYNC;
        op->op.processor = cachefiles_read_copier;

        pagevec_init(&pagevec, 0);

        /* we assume the absence or presence of the first block is a good
         * enough indication for the page as a whole
         * - TODO: don't use bmap() for this as it is _not_ actually good
         *   enough for this as it doesn't indicate errors, but it's all we've
         *   got for the moment
         */
        block0 = page->index;
        block0 <<= shift;

        block = inode->i_mapping->a_ops->bmap(inode->i_mapping, block0);
        _debug("%llx -> %llx",
               (unsigned long long) block0,
               (unsigned long long) block);

        if (block) {
                /* submit the apparently valid page to the backing fs to be
                 * read from disk */
                ret = cachefiles_read_backing_file_one(object, op, page,
                                                       &pagevec);
        } else if (cachefiles_has_space(cache, 0, 1) == 0) {
                /* there's space in the cache we can use */
                fscache_mark_page_cached(op, page);
                fscache_retrieval_complete(op, 1);
                ret = -ENODATA;
        } else {
                goto enobufs;
        }

        _leave(" = %d", ret);
        return ret;

enobufs:
        fscache_retrieval_complete(op, 1);
        _leave(" = -ENOBUFS");
        return -ENOBUFS;
}

/*
 * read the corresponding pages to the given set from the backing file
 * - any uncertain pages are simply discarded, to be tried again another time
 */
static int cachefiles_read_backing_file(struct cachefiles_object *object,
                                        struct fscache_retrieval *op,
                                        struct list_head *list)
{
        struct cachefiles_one_read *monitor = NULL;
        struct address_space *bmapping = object->backer->d_inode->i_mapping;
        struct pagevec lru_pvec;
        struct page *newpage = NULL, *netpage, *_n, *backpage = NULL;
        int ret = 0;

        _enter("");

        pagevec_init(&lru_pvec, 0);

        list_for_each_entry_safe(netpage, _n, list, lru) {
                list_del(&netpage->lru);

                _debug("read back %p{%lu,%d}",
                       netpage, netpage->index, page_count(netpage));

                if (!monitor) {
                        monitor = kzalloc(sizeof(*monitor), cachefiles_gfp);
                        if (!monitor)
                                goto nomem;

                        monitor->op = fscache_get_retrieval(op);
                        init_waitqueue_func_entry(&monitor->monitor,
                                                  cachefiles_read_waiter);
                }

                for (;;) {
                        backpage = find_get_page(bmapping, netpage->index);
                        if (backpage)
                                goto backing_page_already_present;

                        if (!newpage) {
                                newpage = __page_cache_alloc(cachefiles_gfp |
                                                             __GFP_COLD);
                                if (!newpage)
                                        goto nomem;
                        }

                        ret = add_to_page_cache(newpage, bmapping,
                                                netpage->index, cachefiles_gfp);
                        if (ret == 0)
                                goto installed_new_backing_page;
                        if (ret != -EEXIST)
                                goto nomem;
                }

                /* we've installed a new backing page, so now we need to add it
                 * to the LRU list and start it reading */
        installed_new_backing_page:
                _debug("- new %p", newpage);

                backpage = newpage;
                newpage = NULL;

                page_cache_get(backpage);
                if (!pagevec_add(&lru_pvec, backpage))
                        __pagevec_lru_add_file(&lru_pvec);

        reread_backing_page:
                ret = bmapping->a_ops->readpage(NULL, backpage);
                if (ret < 0)
                        goto read_error;

                /* add the netfs page to the pagecache and LRU, and set the
                 * monitor to transfer the data across */
        monitor_backing_page:
                _debug("- monitor add");

                ret = add_to_page_cache(netpage, op->mapping, netpage->index,
                                        cachefiles_gfp);
                if (ret < 0) {
                        if (ret == -EEXIST) {
                                page_cache_release(netpage);
                                fscache_retrieval_complete(op, 1);
                                continue;
                        }
                        goto nomem;
                }

                page_cache_get(netpage);
                if (!pagevec_add(&lru_pvec, netpage))
                        __pagevec_lru_add_file(&lru_pvec);

                /* install a monitor */
                page_cache_get(netpage);
                monitor->netfs_page = netpage;

                page_cache_get(backpage);
                monitor->back_page = backpage;
                monitor->monitor.private = backpage;
                add_page_wait_queue(backpage, &monitor->monitor);
                monitor = NULL;

                /* but the page may have been read before the monitor was
                 * installed, so the monitor may miss the event - so we have to
                 * ensure that we do get one in such a case */
                if (trylock_page(backpage)) {
                        _debug("2unlock %p {%lx}", backpage, backpage->flags);
                        unlock_page(backpage);
                }

                page_cache_release(backpage);
                backpage = NULL;

                page_cache_release(netpage);
                netpage = NULL;
                continue;

                /* if the backing page is already present, it can be in one of
                 * three states: read in progress, read failed or read okay */
        backing_page_already_present:
                _debug("- present %p", backpage);

                if (PageError(backpage))
                        goto io_error;

                if (PageUptodate(backpage))
                        goto backing_page_already_uptodate;

                _debug("- not ready %p{%lx}", backpage, backpage->flags);

                if (!trylock_page(backpage))
                        goto monitor_backing_page;

                if (PageError(backpage)) {
                        _debug("error %lx", backpage->flags);
                        unlock_page(backpage);
                        goto io_error;
                }

                if (PageUptodate(backpage))
                        goto backing_page_already_uptodate_unlock;

                /* we've locked a page that's neither up to date nor erroneous,
                 * so we need to attempt to read it again */
                goto reread_backing_page;

                /* the backing page is already up to date, attach the netfs
                 * page to the pagecache and LRU and copy the data across */
        backing_page_already_uptodate_unlock:
                _debug("uptodate %lx", backpage->flags);
                unlock_page(backpage);
        backing_page_already_uptodate:
                _debug("- uptodate");

                ret = add_to_page_cache(netpage, op->mapping, netpage->index,
                                        cachefiles_gfp);
                if (ret < 0) {
                        if (ret == -EEXIST) {
                                page_cache_release(netpage);
                                fscache_retrieval_complete(op, 1);
                                continue;
                        }
                        goto nomem;
                }

                copy_highpage(netpage, backpage);

                page_cache_release(backpage);
                backpage = NULL;

                fscache_mark_page_cached(op, netpage);

                page_cache_get(netpage);
                if (!pagevec_add(&lru_pvec, netpage))
                        __pagevec_lru_add_file(&lru_pvec);

                /* the netpage is unlocked and marked up to date here */
                fscache_end_io(op, netpage, 0);
                page_cache_release(netpage);
                netpage = NULL;
                fscache_retrieval_complete(op, 1);
                continue;
        }

        netpage = NULL;

        _debug("out");

out:
        /* tidy up */
        pagevec_lru_add_file(&lru_pvec);

        if (newpage)
                page_cache_release(newpage);
        if (netpage)
                page_cache_release(netpage);
        if (backpage)
                page_cache_release(backpage);
        if (monitor) {
                fscache_put_retrieval(op);
                kfree(monitor);
        }

        list_for_each_entry_safe(netpage, _n, list, lru) {
                list_del(&netpage->lru);
                page_cache_release(netpage);
                fscache_retrieval_complete(op, 1);
        }

        _leave(" = %d", ret);
        return ret;

nomem:
        _debug("nomem");
        ret = -ENOMEM;
        goto record_page_complete;

read_error:
        _debug("read error %d", ret);
        if (ret == -ENOMEM)
                goto record_page_complete;
io_error:
        cachefiles_io_error_obj(object, "Page read error on backing file");
        ret = -ENOBUFS;
record_page_complete:
        fscache_retrieval_complete(op, 1);
        goto out;
}

/*
 * read a list of pages from the cache or allocate blocks in which to store
 * them
 */
int cachefiles_read_or_alloc_pages(struct fscache_retrieval *op,
                                   struct list_head *pages,
                                   unsigned *nr_pages,
                                   gfp_t gfp)
{
        struct cachefiles_object *object;
        struct cachefiles_cache *cache;
        struct list_head backpages;
        struct pagevec pagevec;
        struct inode *inode;
        struct page *page, *_n;
        unsigned shift, nrbackpages;
        int ret, ret2, space;

        object = container_of(op->op.object,
                              struct cachefiles_object, fscache);
        cache = container_of(object->fscache.cache,
                             struct cachefiles_cache, cache);

        _enter("{OBJ%x,%d},,%d,,",
               object->fscache.debug_id, atomic_read(&op->op.usage),
               *nr_pages);

        if (!object->backer)
                goto all_enobufs;

        space = 1;
        if (cachefiles_has_space(cache, 0, *nr_pages) < 0)
                space = 0;

        inode = object->backer->d_inode;
        ASSERT(S_ISREG(inode->i_mode));
        ASSERT(inode->i_mapping->a_ops->bmap);
        ASSERT(inode->i_mapping->a_ops->readpages);

        /* calculate the shift required to use bmap */
        if (inode->i_sb->s_blocksize > PAGE_SIZE)
                goto all_enobufs;

        shift = PAGE_SHIFT - inode->i_sb->s_blocksize_bits;

        pagevec_init(&pagevec, 0);

        op->op.flags &= FSCACHE_OP_KEEP_FLAGS;
        op->op.flags |= FSCACHE_OP_ASYNC;
        op->op.processor = cachefiles_read_copier;

        INIT_LIST_HEAD(&backpages);
        nrbackpages = 0;

        ret = space ? -ENODATA : -ENOBUFS;
        list_for_each_entry_safe(page, _n, pages, lru) {
                sector_t block0, block;

                /* we assume the absence or presence of the first block is a
                 * good enough indication for the page as a whole
                 * - TODO: don't use bmap() for this as it is _not_ actually
                 *   good enough for this as it doesn't indicate errors, but
                 *   it's all we've got for the moment
                 */
                block0 = page->index;
                block0 <<= shift;

                block = inode->i_mapping->a_ops->bmap(inode->i_mapping,
                                                      block0);
                _debug("%llx -> %llx",
                       (unsigned long long) block0,
                       (unsigned long long) block);

                if (block) {
                        /* we have data - add it to the list to give to the
                         * backing fs */
                        list_move(&page->lru, &backpages);
                        (*nr_pages)--;
                        nrbackpages++;
                } else if (space && pagevec_add(&pagevec, page) == 0) {
                        fscache_mark_pages_cached(op, &pagevec);
                        fscache_retrieval_complete(op, 1);
                        ret = -ENODATA;
                } else {
                        fscache_retrieval_complete(op, 1);
                }
        }

        if (pagevec_count(&pagevec) > 0)
                fscache_mark_pages_cached(op, &pagevec);

        if (list_empty(pages))
                ret = 0;

        /* submit the apparently valid pages to the backing fs to be read from
         * disk */
        if (nrbackpages > 0) {
                ret2 = cachefiles_read_backing_file(object, op, &backpages);
                if (ret2 == -ENOMEM || ret2 == -EINTR)
                        ret = ret2;
        }

        _leave(" = %d [nr=%u%s]",
               ret, *nr_pages, list_empty(pages) ? " empty" : "");
        return ret;

all_enobufs:
        fscache_retrieval_complete(op, *nr_pages);
        return -ENOBUFS;
}

/*
 * allocate a block in the cache in which to store a page
 * - cache withdrawal is prevented by the caller
 * - returns -EINTR if interrupted
 * - returns -ENOMEM if ran out of memory
 * - returns -ENOBUFS if no buffers can be made available
 * - returns -ENOBUFS if page is beyond EOF
 * - otherwise:
 *   - the metadata will be retained
 *   - 0 will be returned
 */
int cachefiles_allocate_page(struct fscache_retrieval *op,
                             struct page *page,
                             gfp_t gfp)
{
        struct cachefiles_object *object;
        struct cachefiles_cache *cache;
        int ret;

        object = container_of(op->op.object,
                              struct cachefiles_object, fscache);
        cache = container_of(object->fscache.cache,
                             struct cachefiles_cache, cache);

        _enter("%p,{%lx},", object, page->index);

        ret = cachefiles_has_space(cache, 0, 1);
        if (ret == 0)
                fscache_mark_page_cached(op, page);
        else
                ret = -ENOBUFS;

        fscache_retrieval_complete(op, 1);
        _leave(" = %d", ret);
        return ret;
}

/*
 * allocate blocks in the cache in which to store a set of pages
 * - cache withdrawal is prevented by the caller
 * - returns -EINTR if interrupted
 * - returns -ENOMEM if ran out of memory
 * - returns -ENOBUFS if some buffers couldn't be made available
 * - returns -ENOBUFS if some pages are beyond EOF
 * - otherwise:
 *   - -ENODATA will be returned
 * - metadata will be retained for any page marked
 */
int cachefiles_allocate_pages(struct fscache_retrieval *op,
                              struct list_head *pages,
                              unsigned *nr_pages,
                              gfp_t gfp)
{
        struct cachefiles_object *object;
        struct cachefiles_cache *cache;
        struct pagevec pagevec;
        struct page *page;
        int ret;

        object = container_of(op->op.object,
                              struct cachefiles_object, fscache);
        cache = container_of(object->fscache.cache,
                             struct cachefiles_cache, cache);

        _enter("%p,,,%d,", object, *nr_pages);

        ret = cachefiles_has_space(cache, 0, *nr_pages);
        if (ret == 0) {
                pagevec_init(&pagevec, 0);

                list_for_each_entry(page, pages, lru) {
                        if (pagevec_add(&pagevec, page) == 0)
                                fscache_mark_pages_cached(op, &pagevec);
                }

                if (pagevec_count(&pagevec) > 0)
                        fscache_mark_pages_cached(op, &pagevec);
                ret = -ENODATA;
        } else {
                ret = -ENOBUFS;
        }

        fscache_retrieval_complete(op, *nr_pages);
        _leave(" = %d", ret);
        return ret;
}

/*
 * request a page be stored in the cache
 * - cache withdrawal is prevented by the caller
 * - this request may be ignored if there's no cache block available, in which
 *   case -ENOBUFS will be returned
 * - if the op is in progress, 0 will be returned
 */
int cachefiles_write_page(struct fscache_storage *op, struct page *page)
{
        struct cachefiles_object *object;
        struct cachefiles_cache *cache;
        mm_segment_t old_fs;
        struct file *file;
        struct path path;
        loff_t pos, eof;
        size_t len;
        void *data;
        int ret;

        ASSERT(op != NULL);
        ASSERT(page != NULL);

        object = container_of(op->op.object,
                              struct cachefiles_object, fscache);

        _enter("%p,%p{%lx},,,", object, page, page->index);

        if (!object->backer) {
                _leave(" = -ENOBUFS");
                return -ENOBUFS;
        }

        ASSERT(S_ISREG(object->backer->d_inode->i_mode));

        cache = container_of(object->fscache.cache,
                             struct cachefiles_cache, cache);

        /* write the page to the backing filesystem and let it store it in its
         * own time */
        path.mnt = cache->mnt;
        path.dentry = object->backer;
        file = dentry_open(&path, O_RDWR | O_LARGEFILE, cache->cache_cred);
        if (IS_ERR(file)) {
                ret = PTR_ERR(file);
        } else {
                ret = -EIO;
                if (file->f_op->write) {
                        pos = (loff_t) page->index << PAGE_SHIFT;

                        /* we mustn't write more data than we have, so we have
                         * to beware of a partial page at EOF */
                        eof = object->fscache.store_limit_l;
                        len = PAGE_SIZE;
                        if (eof & ~PAGE_MASK) {
                                ASSERTCMP(pos, <, eof);
                                if (eof - pos < PAGE_SIZE) {
                                        _debug("cut short %llx to %llx",
                                               pos, eof);
                                        len = eof - pos;
                                        ASSERTCMP(pos + len, ==, eof);
                                }
                        }

                        data = kmap(page);
                        file_start_write(file);
                        old_fs = get_fs();
                        set_fs(KERNEL_DS);
                        ret = file->f_op->write(
                                file, (const void __user *) data, len, &pos);
                        set_fs(old_fs);
                        kunmap(page);
                        file_end_write(file);
                        if (ret != len)
                                ret = -EIO;
                }
                fput(file);
        }

        if (ret < 0) {
                if (ret == -EIO)
                        cachefiles_io_error_obj(
                                object, "Write page to backing file failed");
                ret = -ENOBUFS;
        }

        _leave(" = %d", ret);
        return ret;
}

/*
 * detach a backing block from a page
 * - cache withdrawal is prevented by the caller
 */
void cachefiles_uncache_page(struct fscache_object *_object, struct page *page)
{
        struct cachefiles_object *object;
        struct cachefiles_cache *cache;

        object = container_of(_object, struct cachefiles_object, fscache);
        cache = container_of(object->fscache.cache,

                             struct cachefiles_cache, cache);

        _enter("%p,{%lu}", object, page->index);

        spin_unlock(&object->fscache.cookie->lock);
}