# SPDX-License-Identifier: GPL-2.0-only

menu "Memory Management options"

config SELECT_MEMORY_MODEL
        def_bool y
        depends on ARCH_SELECT_MEMORY_MODEL

choice
        prompt "Memory model"
        depends on SELECT_MEMORY_MODEL
        default DISCONTIGMEM_MANUAL if ARCH_DISCONTIGMEM_DEFAULT
        default SPARSEMEM_MANUAL if ARCH_SPARSEMEM_DEFAULT
        default FLATMEM_MANUAL
        help
          This option allows you to change some of the ways that
          Linux manages its memory internally. Most users will
          only have one option here selected by the architecture
          configuration. This is normal.

config FLATMEM_MANUAL
        bool "Flat Memory"
        depends on !(ARCH_DISCONTIGMEM_ENABLE || ARCH_SPARSEMEM_ENABLE) || ARCH_FLATMEM_ENABLE
        help
          This option is best suited for non-NUMA systems with
          flat address space. The FLATMEM is the most efficient
          system in terms of performance and resource consumption
          and it is the best option for smaller systems.

          For systems that have holes in their physical address
          spaces and for features like NUMA and memory hotplug,
          choose "Sparse Memory"

          If unsure, choose this option (Flat Memory) over any other.

config DISCONTIGMEM_MANUAL
        bool "Discontiguous Memory"
        depends on ARCH_DISCONTIGMEM_ENABLE
        help
          This option provides enhanced support for discontiguous
          memory systems, over FLATMEM.  These systems have holes
          in their physical address spaces, and this option provides
          more efficient handling of these holes.

          Although "Discontiguous Memory" is still used by several
          architectures, it is considered deprecated in favor of
          "Sparse Memory".

          If unsure, choose "Sparse Memory" over this option.

config SPARSEMEM_MANUAL
        bool "Sparse Memory"
        depends on ARCH_SPARSEMEM_ENABLE
        help
          This will be the only option for some systems, including
          memory hot-plug systems.  This is normal.

          This option provides efficient support for systems with
          holes is their physical address space and allows memory
          hot-plug and hot-remove.

          If unsure, choose "Flat Memory" over this option.

endchoice

config DISCONTIGMEM
        def_bool y
        depends on (!SELECT_MEMORY_MODEL && ARCH_DISCONTIGMEM_ENABLE) || DISCONTIGMEM_MANUAL

config SPARSEMEM
        def_bool y
        depends on (!SELECT_MEMORY_MODEL && ARCH_SPARSEMEM_ENABLE) || SPARSEMEM_MANUAL

config FLATMEM
        def_bool y
        depends on (!DISCONTIGMEM && !SPARSEMEM) || FLATMEM_MANUAL

config FLAT_NODE_MEM_MAP
        def_bool y
        depends on !SPARSEMEM

#
# Both the NUMA code and DISCONTIGMEM use arrays of pg_data_t's
# to represent different areas of memory.  This variable allows
# those dependencies to exist individually.
#
config NEED_MULTIPLE_NODES
        def_bool y
        depends on DISCONTIGMEM || NUMA

config HAVE_MEMORY_PRESENT
        def_bool y
        depends on ARCH_HAVE_MEMORY_PRESENT || SPARSEMEM

#
# SPARSEMEM_EXTREME (which is the default) does some bootmem
# allocations when memory_present() is called.  If this cannot
# be done on your architecture, select this option.  However,
# statically allocating the mem_section[] array can potentially
# consume vast quantities of .bss, so be careful.
#
# This option will also potentially produce smaller runtime code
# with gcc 3.4 and later.
#
config SPARSEMEM_STATIC
        bool

#
# Architecture platforms which require a two level mem_section in SPARSEMEM
# must select this option. This is usually for architecture platforms with
# an extremely sparse physical address space.
#
config SPARSEMEM_EXTREME
        def_bool y
        depends on SPARSEMEM && !SPARSEMEM_STATIC

config SPARSEMEM_VMEMMAP_ENABLE
        bool

config SPARSEMEM_VMEMMAP
        bool "Sparse Memory virtual memmap"
        depends on SPARSEMEM && SPARSEMEM_VMEMMAP_ENABLE
        default y
        help
         SPARSEMEM_VMEMMAP uses a virtually mapped memmap to optimise
         pfn_to_page and page_to_pfn operations.  This is the most
         efficient option when sufficient kernel resources are available.

config HAVE_MEMBLOCK_NODE_MAP
        bool

config HAVE_MEMBLOCK_PHYS_MAP
        bool

config HAVE_FAST_GUP
        depends on MMU
        bool

config ARCH_KEEP_MEMBLOCK
        bool

config MEMORY_ISOLATION
        bool

#
# Only be set on architectures that have completely implemented memory hotplug
# feature. If you are not sure, don't touch it.
#
config HAVE_BOOTMEM_INFO_NODE
        def_bool n

# eventually, we can have this option just 'select SPARSEMEM'
config MEMORY_HOTPLUG
        bool "Allow for memory hot-add"
        depends on SPARSEMEM || X86_64_ACPI_NUMA
        depends on ARCH_ENABLE_MEMORY_HOTPLUG

config MEMORY_HOTPLUG_SPARSE
        def_bool y
        depends on SPARSEMEM && MEMORY_HOTPLUG

config MEMORY_HOTPLUG_DEFAULT_ONLINE
        bool "Online the newly added memory blocks by default"
        depends on MEMORY_HOTPLUG
        help
          This option sets the default policy setting for memory hotplug
          onlining policy (/sys/devices/system/memory/auto_online_blocks) which
          determines what happens to newly added memory regions. Policy setting
          can always be changed at runtime.
          See Documentation/admin-guide/mm/memory-hotplug.rst for more information.

          Say Y here if you want all hot-plugged memory blocks to appear in
          'online' state by default.
          Say N here if you want the default policy to keep all hot-plugged
          memory blocks in 'offline' state.

config MEMORY_HOTREMOVE
        bool "Allow for memory hot remove"
        select MEMORY_ISOLATION
        select HAVE_BOOTMEM_INFO_NODE if (X86_64 || PPC64)
        depends on MEMORY_HOTPLUG && ARCH_ENABLE_MEMORY_HOTREMOVE
        depends on MIGRATION

# Heavily threaded applications may benefit from splitting the mm-wide
# page_table_lock, so that faults on different parts of the user address
# space can be handled with less contention: split it at this NR_CPUS.
# Default to 4 for wider testing, though 8 might be more appropriate.
# ARM's adjust_pte (unused if VIPT) depends on mm-wide page_table_lock.
# PA-RISC 7xxx's spinlock_t would enlarge struct page from 32 to 44 bytes.
# DEBUG_SPINLOCK and DEBUG_LOCK_ALLOC spinlock_t also enlarge struct page.
#
config SPLIT_PTLOCK_CPUS
        int
        default "999999" if !MMU
        default "999999" if ARM && !CPU_CACHE_VIPT
        default "999999" if PARISC && !PA20
        default "4"

config ARCH_ENABLE_SPLIT_PMD_PTLOCK
        bool

#
# support for memory balloon
config MEMORY_BALLOON
        bool

#
# support for memory balloon compaction
config BALLOON_COMPACTION
        bool "Allow for balloon memory compaction/migration"
        def_bool y
        depends on COMPACTION && MEMORY_BALLOON
        help
          Memory fragmentation introduced by ballooning might reduce
          significantly the number of 2MB contiguous memory blocks that can be
          used within a guest, thus imposing performance penalties associated
          with the reduced number of transparent huge pages that could be used
          by the guest workload. Allowing the compaction & migration for memory
          pages enlisted as being part of memory balloon devices avoids the
          scenario aforementioned and helps improving memory defragmentation.

#
# support for memory compaction
config COMPACTION
        bool "Allow for memory compaction"
        def_bool y
        select MIGRATION
        depends on MMU
        help
          Compaction is the only memory management component to form
          high order (larger physically contiguous) memory blocks
          reliably. The page allocator relies on compaction heavily and
          the lack of the feature can lead to unexpected OOM killer
          invocations for high order memory requests. You shouldn't
          disable this option unless there really is a strong reason for
          it and then we would be really interested to hear about that at
          linux-mm@kvack.org.

#
# support for page migration
#
config MIGRATION
        bool "Page migration"
        def_bool y
        depends on (NUMA || ARCH_ENABLE_MEMORY_HOTREMOVE || COMPACTION || CMA) && MMU
        help
          Allows the migration of the physical location of pages of processes
          while the virtual addresses are not changed. This is useful in
          two situations. The first is on NUMA systems to put pages nearer
          to the processors accessing. The second is when allocating huge
          pages as migration can relocate pages to satisfy a huge page
          allocation instead of reclaiming.

config ARCH_ENABLE_HUGEPAGE_MIGRATION
        bool

config ARCH_ENABLE_THP_MIGRATION
        bool

config CONTIG_ALLOC
       def_bool (MEMORY_ISOLATION && COMPACTION) || CMA

config PHYS_ADDR_T_64BIT
        def_bool 64BIT

config BOUNCE
        bool "Enable bounce buffers"
        default y
        depends on BLOCK && MMU && (ZONE_DMA || HIGHMEM)
        help
          Enable bounce buffers for devices that cannot access
          the full range of memory available to the CPU. Enabled
          by default when ZONE_DMA or HIGHMEM is selected, but you
          may say n to override this.

config NR_QUICK
        int
        depends on QUICKLIST
        default "1"

config VIRT_TO_BUS
        bool
        help
          An architecture should select this if it implements the
          deprecated interface virt_to_bus().  All new architectures
          should probably not select this.


config MMU_NOTIFIER
        bool
        select SRCU

config KSM
        bool "Enable KSM for page merging"
        depends on MMU
        select XXHASH
        help
          Enable Kernel Samepage Merging: KSM periodically scans those areas
          of an application's address space that an app has advised may be
          mergeable.  When it finds pages of identical content, it replaces
          the many instances by a single page with that content, so
          saving memory until one or another app needs to modify the content.
          Recommended for use with KVM, or with other duplicative applications.
          See Documentation/vm/ksm.rst for more information: KSM is inactive
          until a program has madvised that an area is MADV_MERGEABLE, and
          root has set /sys/kernel/mm/ksm/run to 1 (if CONFIG_SYSFS is set).

config DEFAULT_MMAP_MIN_ADDR
        int "Low address space to protect from user allocation"
        depends on MMU
        default 4096
        help
          This is the portion of low virtual memory which should be protected
          from userspace allocation.  Keeping a user from writing to low pages
          can help reduce the impact of kernel NULL pointer bugs.

          For most ia64, ppc64 and x86 users with lots of address space
          a value of 65536 is reasonable and should cause no problems.
          On arm and other archs it should not be higher than 32768.
          Programs which use vm86 functionality or have some need to map
          this low address space will need CAP_SYS_RAWIO or disable this
          protection by setting the value to 0.

          This value can be changed after boot using the
          /proc/sys/vm/mmap_min_addr tunable.

config ARCH_SUPPORTS_MEMORY_FAILURE
        bool

config MEMORY_FAILURE
        depends on MMU
        depends on ARCH_SUPPORTS_MEMORY_FAILURE
        bool "Enable recovery from hardware memory errors"
        select MEMORY_ISOLATION
        select RAS
        help
          Enables code to recover from some memory failures on systems
          with MCA recovery. This allows a system to continue running
          even when some of its memory has uncorrected errors. This requires
          special hardware support and typically ECC memory.

config HWPOISON_INJECT
        tristate "HWPoison pages injector"
        depends on MEMORY_FAILURE && DEBUG_KERNEL && PROC_FS
        select PROC_PAGE_MONITOR

config NOMMU_INITIAL_TRIM_EXCESS
        int "Turn on mmap() excess space trimming before booting"
        depends on !MMU
        default 1
        help
          The NOMMU mmap() frequently needs to allocate large contiguous chunks
          of memory on which to store mappings, but it can only ask the system
          allocator for chunks in 2^N*PAGE_SIZE amounts - which is frequently
          more than it requires.  To deal with this, mmap() is able to trim off
          the excess and return it to the allocator.

          If trimming is enabled, the excess is trimmed off and returned to the
          system allocator, which can cause extra fragmentation, particularly
          if there are a lot of transient processes.

          If trimming is disabled, the excess is kept, but not used, which for
          long-term mappings means that the space is wasted.

          Trimming can be dynamically controlled through a sysctl option
          (/proc/sys/vm/nr_trim_pages) which specifies the minimum number of
          excess pages there must be before trimming should occur, or zero if
          no trimming is to occur.

          This option specifies the initial value of this option.  The default
          of 1 says that all excess pages should be trimmed.

          See Documentation/nommu-mmap.txt for more information.

config TRANSPARENT_HUGEPAGE
        bool "Transparent Hugepage Support"
        depends on HAVE_ARCH_TRANSPARENT_HUGEPAGE
        select COMPACTION
        select XARRAY_MULTI
        help
          Transparent Hugepages allows the kernel to use huge pages and
          huge tlb transparently to the applications whenever possible.
          This feature can improve computing performance to certain
          applications by speeding up page faults during memory
          allocation, by reducing the number of tlb misses and by speeding
          up the pagetable walking.

          If memory constrained on embedded, you may want to say N.

choice
        prompt "Transparent Hugepage Support sysfs defaults"
        depends on TRANSPARENT_HUGEPAGE
        default TRANSPARENT_HUGEPAGE_ALWAYS
        help
          Selects the sysfs defaults for Transparent Hugepage Support.

        config TRANSPARENT_HUGEPAGE_ALWAYS
                bool "always"
        help
          Enabling Transparent Hugepage always, can increase the
          memory footprint of applications without a guaranteed
          benefit but it will work automatically for all applications.

        config TRANSPARENT_HUGEPAGE_MADVISE
                bool "madvise"
        help
          Enabling Transparent Hugepage madvise, will only provide a
          performance improvement benefit to the applications using
          madvise(MADV_HUGEPAGE) but it won't risk to increase the
          memory footprint of applications without a guaranteed
          benefit.
endchoice

config ARCH_WANTS_THP_SWAP
       def_bool n

config THP_SWAP
        def_bool y
        depends on TRANSPARENT_HUGEPAGE && ARCH_WANTS_THP_SWAP && SWAP
        help
          Swap transparent huge pages in one piece, without splitting.
          XXX: For now, swap cluster backing transparent huge page
          will be split after swapout.

          For selection by architectures with reasonable THP sizes.

config  TRANSPARENT_HUGE_PAGECACHE
        def_bool y
        depends on TRANSPARENT_HUGEPAGE

#
# UP and nommu archs use km based percpu allocator
#
config NEED_PER_CPU_KM
        depends on !SMP
        bool
        default y

config CLEANCACHE
        bool "Enable cleancache driver to cache clean pages if tmem is present"
        help
          Cleancache can be thought of as a page-granularity victim cache
          for clean pages that the kernel's pageframe replacement algorithm
          (PFRA) would like to keep around, but can't since there isn't enough
          memory.  So when the PFRA "evicts" a page, it first attempts to use
          cleancache code to put the data contained in that page into
          "transcendent memory", memory that is not directly accessible or
          addressable by the kernel and is of unknown and possibly
          time-varying size.  And when a cleancache-enabled
          filesystem wishes to access a page in a file on disk, it first
          checks cleancache to see if it already contains it; if it does,
          the page is copied into the kernel and a disk access is avoided.
          When a transcendent memory driver is available (such as zcache or
          Xen transcendent memory), a significant I/O reduction
          may be achieved.  When none is available, all cleancache calls
          are reduced to a single pointer-compare-against-NULL resulting
          in a negligible performance hit.

          If unsure, say Y to enable cleancache

config FRONTSWAP
        bool "Enable frontswap to cache swap pages if tmem is present"
        depends on SWAP
        help
          Frontswap is so named because it can be thought of as the opposite
          of a "backing" store for a swap device.  The data is stored into
          "transcendent memory", memory that is not directly accessible or
          addressable by the kernel and is of unknown and possibly
          time-varying size.  When space in transcendent memory is available,
          a significant swap I/O reduction may be achieved.  When none is
          available, all frontswap calls are reduced to a single pointer-
          compare-against-NULL resulting in a negligible performance hit
          and swap data is stored as normal on the matching swap device.

          If unsure, say Y to enable frontswap.

config CMA
        bool "Contiguous Memory Allocator"
        depends on MMU
        select MIGRATION
        select MEMORY_ISOLATION
        help
          This enables the Contiguous Memory Allocator which allows other
          subsystems to allocate big physically-contiguous blocks of memory.
          CMA reserves a region of memory and allows only movable pages to
          be allocated from it. This way, the kernel can use the memory for
          pagecache and when a subsystem requests for contiguous area, the
          allocated pages are migrated away to serve the contiguous request.

          If unsure, say "n".

config CMA_DEBUG
        bool "CMA debug messages (DEVELOPMENT)"
        depends on DEBUG_KERNEL && CMA
        help
          Turns on debug messages in CMA.  This produces KERN_DEBUG
          messages for every CMA call as well as various messages while
          processing calls such as dma_alloc_from_contiguous().
          This option does not affect warning and error messages.

config CMA_DEBUGFS
        bool "CMA debugfs interface"
        depends on CMA && DEBUG_FS
        help
          Turns on the DebugFS interface for CMA.

config CMA_AREAS
        int "Maximum count of the CMA areas"
        depends on CMA
        default 7
        help
          CMA allows to create CMA areas for particular purpose, mainly,
          used as device private area. This parameter sets the maximum
          number of CMA area in the system.

          If unsure, leave the default value "7".

config MEM_SOFT_DIRTY
        bool "Track memory changes"
        depends on CHECKPOINT_RESTORE && HAVE_ARCH_SOFT_DIRTY && PROC_FS
        select PROC_PAGE_MONITOR
        help
          This option enables memory changes tracking by introducing a
          soft-dirty bit on pte-s. This bit it set when someone writes
          into a page just as regular dirty bit, but unlike the latter
          it can be cleared by hands.

          See Documentation/admin-guide/mm/soft-dirty.rst for more details.

config ZSWAP
        bool "Compressed cache for swap pages (EXPERIMENTAL)"
        depends on FRONTSWAP && CRYPTO=y
        select CRYPTO_LZO
        select ZPOOL
        help
          A lightweight compressed cache for swap pages.  It takes
          pages that are in the process of being swapped out and attempts to
          compress them into a dynamically allocated RAM-based memory pool.
          This can result in a significant I/O reduction on swap device and,
          in the case where decompressing from RAM is faster that swap device
          reads, can also improve workload performance.

          This is marked experimental because it is a new feature (as of
          v3.11) that interacts heavily with memory reclaim.  While these
          interactions don't cause any known issues on simple memory setups,
          they have not be fully explored on the large set of potential
          configurations and workloads that exist.

config ZPOOL
        tristate "Common API for compressed memory storage"
        help
          Compressed memory storage API.  This allows using either zbud or
          zsmalloc.

config ZBUD
        tristate "Low (Up to 2x) density storage for compressed pages"
        help
          A special purpose allocator for storing compressed pages.
          It is designed to store up to two compressed pages per physical
          page.  While this design limits storage density, it has simple and
          deterministic reclaim properties that make it preferable to a higher
          density approach when reclaim will be used.

config Z3FOLD
        tristate "Up to 3x density storage for compressed pages"
        depends on ZPOOL
        help
          A special purpose allocator for storing compressed pages.
          It is designed to store up to three compressed pages per physical
          page. It is a ZBUD derivative so the simplicity and determinism are
          still there.

config ZSMALLOC
        tristate "Memory allocator for compressed pages"
        depends on MMU
        help
          zsmalloc is a slab-based memory allocator designed to store
          compressed RAM pages.  zsmalloc uses virtual memory mapping
          in order to reduce fragmentation.  However, this results in a
          non-standard allocator interface where a handle, not a pointer, is
          returned by an alloc().  This handle must be mapped in order to
          access the allocated space.

config PGTABLE_MAPPING
        bool "Use page table mapping to access object in zsmalloc"
        depends on ZSMALLOC
        help
          By default, zsmalloc uses a copy-based object mapping method to
          access allocations that span two pages. However, if a particular
          architecture (ex, ARM) performs VM mapping faster than copying,
          then you should select this. This causes zsmalloc to use page table
          mapping rather than copying for object mapping.

          You can check speed with zsmalloc benchmark:
          https://github.com/spartacus06/zsmapbench

config ZSMALLOC_STAT
        bool "Export zsmalloc statistics"
        depends on ZSMALLOC
        select DEBUG_FS
        help
          This option enables code in the zsmalloc to collect various
          statistics about whats happening in zsmalloc and exports that
          information to userspace via debugfs.
          If unsure, say N.

config GENERIC_EARLY_IOREMAP
        bool

config MAX_STACK_SIZE_MB
        int "Maximum user stack size for 32-bit processes (MB)"
        default 80
        range 8 2048
        depends on STACK_GROWSUP && (!64BIT || COMPAT)
        help
          This is the maximum stack size in Megabytes in the VM layout of 32-bit
          user processes when the stack grows upwards (currently only on parisc
          arch). The stack will be located at the highest memory address minus
          the given value, unless the RLIMIT_STACK hard limit is changed to a
          smaller value in which case that is used.

          A sane initial value is 80 MB.

config DEFERRED_STRUCT_PAGE_INIT
        bool "Defer initialisation of struct pages to kthreads"
        depends on SPARSEMEM
        depends on !NEED_PER_CPU_KM
        depends on 64BIT
        help
          Ordinarily all struct pages are initialised during early boot in a
          single thread. On very large machines this can take a considerable
          amount of time. If this option is set, large machines will bring up
          a subset of memmap at boot and then initialise the rest in parallel
          by starting one-off "pgdatinitX" kernel thread for each node X. This
          has a potential performance impact on processes running early in the
          lifetime of the system until these kthreads finish the
          initialisation.

config IDLE_PAGE_TRACKING
        bool "Enable idle page tracking"
        depends on SYSFS && MMU
        select PAGE_EXTENSION if !64BIT
        help
          This feature allows to estimate the amount of user pages that have
          not been touched during a given period of time. This information can
          be useful to tune memory cgroup limits and/or for job placement
          within a compute cluster.

          See Documentation/admin-guide/mm/idle_page_tracking.rst for
          more details.

config ARCH_HAS_PTE_DEVMAP
        bool

config ZONE_DEVICE
        bool "Device memory (pmem, HMM, etc...) hotplug support"
        depends on MEMORY_HOTPLUG
        depends on MEMORY_HOTREMOVE
        depends on SPARSEMEM_VMEMMAP
        depends on ARCH_HAS_PTE_DEVMAP
        select XARRAY_MULTI

        help
          Device memory hotplug support allows for establishing pmem,
          or other device driver discovered memory regions, in the
          memmap. This allows pfn_to_page() lookups of otherwise
          "device-physical" addresses which is needed for using a DAX
          mapping in an O_DIRECT operation, among other things.

          If FS_DAX is enabled, then say Y.

config MIGRATE_VMA_HELPER
        bool

config DEV_PAGEMAP_OPS
        bool

config HMM_MIRROR
        bool "HMM mirror CPU page table into a device page table"
        depends on (X86_64 || PPC64)
        depends on MMU && 64BIT
        select MMU_NOTIFIER
        help
          Select HMM_MIRROR if you want to mirror range of the CPU page table of a
          process into a device page table. Here, mirror means "keep synchronized".
          Prerequisites: the device must provide the ability to write-protect its
          page tables (at PAGE_SIZE granularity), and must be able to recover from
          the resulting potential page faults.

config DEVICE_PRIVATE
        bool "Unaddressable device memory (GPU memory, ...)"
        depends on ZONE_DEVICE
        select DEV_PAGEMAP_OPS

        help
          Allows creation of struct pages to represent unaddressable device
          memory; i.e., memory that is only accessible from the device (or
          group of devices). You likely also want to select HMM_MIRROR.

config FRAME_VECTOR
        bool

config ARCH_USES_HIGH_VMA_FLAGS
        bool
config ARCH_HAS_PKEYS
        bool

config PERCPU_STATS
        bool "Collect percpu memory statistics"
        help
          This feature collects and exposes statistics via debugfs. The
          information includes global and per chunk statistics, which can
          be used to help understand percpu memory usage.

config GUP_BENCHMARK
        bool "Enable infrastructure for get_user_pages_fast() benchmarking"
        help
          Provides /sys/kernel/debug/gup_benchmark that helps with testing
          performance of get_user_pages_fast().

          See tools/testing/selftests/vm/gup_benchmark.c

config GUP_GET_PTE_LOW_HIGH
        bool

config ARCH_HAS_PTE_SPECIAL
        bool

#
# Some architectures require a special hugepage directory format that is
# required to support multiple hugepage sizes. For example a4fe3ce76
# "powerpc/mm: Allow more flexible layouts for hugepage pagetables"
# introduced it on powerpc.  This allows for a more flexible hugepage
# pagetable layouts.
#
config ARCH_HAS_HUGEPD
        bool

endmenu