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3822a7c409
F_SEAL_EXEC") which permits the setting of the memfd execute bit at
memfd creation time, with the option of sealing the state of the X bit.
- Peter Xu adds a patch series ("mm/hugetlb: Make huge_pte_offset()
thread-safe for pmd unshare") which addresses a rare race condition
related to PMD unsharing.
- Several folioification patch serieses from Matthew Wilcox, Vishal
Moola, Sidhartha Kumar and Lorenzo Stoakes
- Johannes Weiner has a series ("mm: push down lock_page_memcg()") which
does perform some memcg maintenance and cleanup work.
- SeongJae Park has added DAMOS filtering to DAMON, with the series
"mm/damon/core: implement damos filter". These filters provide users
with finer-grained control over DAMOS's actions. SeongJae has also done
some DAMON cleanup work.
- Kairui Song adds a series ("Clean up and fixes for swap").
- Vernon Yang contributed the series "Clean up and refinement for maple
tree".
- Yu Zhao has contributed the "mm: multi-gen LRU: memcg LRU" series. It
adds to MGLRU an LRU of memcgs, to improve the scalability of global
reclaim.
- David Hildenbrand has added some userfaultfd cleanup work in the
series "mm: uffd-wp + change_protection() cleanups".
- Christoph Hellwig has removed the generic_writepages() library
function in the series "remove generic_writepages".
- Baolin Wang has performed some maintenance on the compaction code in
his series "Some small improvements for compaction".
- Sidhartha Kumar is doing some maintenance work on struct page in his
series "Get rid of tail page fields".
- David Hildenbrand contributed some cleanup, bugfixing and
generalization of pte management and of pte debugging in his series "mm:
support __HAVE_ARCH_PTE_SWP_EXCLUSIVE on all architectures with swap
PTEs".
- Mel Gorman and Neil Brown have removed the __GFP_ATOMIC allocation
flag in the series "Discard __GFP_ATOMIC".
- Sergey Senozhatsky has improved zsmalloc's memory utilization with his
series "zsmalloc: make zspage chain size configurable".
- Joey Gouly has added prctl() support for prohibiting the creation of
writeable+executable mappings. The previous BPF-based approach had
shortcomings. See "mm: In-kernel support for memory-deny-write-execute
(MDWE)".
- Waiman Long did some kmemleak cleanup and bugfixing in the series
"mm/kmemleak: Simplify kmemleak_cond_resched() & fix UAF".
- T.J. Alumbaugh has contributed some MGLRU cleanup work in his series
"mm: multi-gen LRU: improve".
- Jiaqi Yan has provided some enhancements to our memory error
statistics reporting, mainly by presenting the statistics on a per-node
basis. See the series "Introduce per NUMA node memory error
statistics".
- Mel Gorman has a second and hopefully final shot at fixing a CPU-hog
regression in compaction via his series "Fix excessive CPU usage during
compaction".
- Christoph Hellwig does some vmalloc maintenance work in the series
"cleanup vfree and vunmap".
- Christoph Hellwig has removed block_device_operations.rw_page() in ths
series "remove ->rw_page".
- We get some maple_tree improvements and cleanups in Liam Howlett's
series "VMA tree type safety and remove __vma_adjust()".
- Suren Baghdasaryan has done some work on the maintainability of our
vm_flags handling in the series "introduce vm_flags modifier functions".
- Some pagemap cleanup and generalization work in Mike Rapoport's series
"mm, arch: add generic implementation of pfn_valid() for FLATMEM" and
"fixups for generic implementation of pfn_valid()"
- Baoquan He has done some work to make /proc/vmallocinfo and
/proc/kcore better represent the real state of things in his series
"mm/vmalloc.c: allow vread() to read out vm_map_ram areas".
- Jason Gunthorpe rationalized the GUP system's interface to the rest of
the kernel in the series "Simplify the external interface for GUP".
- SeongJae Park wishes to migrate people from DAMON's debugfs interface
over to its sysfs interface. To support this, we'll temporarily be
printing warnings when people use the debugfs interface. See the series
"mm/damon: deprecate DAMON debugfs interface".
- Andrey Konovalov provided the accurately named "lib/stackdepot: fixes
and clean-ups" series.
- Huang Ying has provided a dramatic reduction in migration's TLB flush
IPI rates with the series "migrate_pages(): batch TLB flushing".
- Arnd Bergmann has some objtool fixups in "objtool warning fixes".
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Merge tag 'mm-stable-2023-02-20-13-37' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
Pull MM updates from Andrew Morton:
- Daniel Verkamp has contributed a memfd series ("mm/memfd: add
F_SEAL_EXEC") which permits the setting of the memfd execute bit at
memfd creation time, with the option of sealing the state of the X
bit.
- Peter Xu adds a patch series ("mm/hugetlb: Make huge_pte_offset()
thread-safe for pmd unshare") which addresses a rare race condition
related to PMD unsharing.
- Several folioification patch serieses from Matthew Wilcox, Vishal
Moola, Sidhartha Kumar and Lorenzo Stoakes
- Johannes Weiner has a series ("mm: push down lock_page_memcg()")
which does perform some memcg maintenance and cleanup work.
- SeongJae Park has added DAMOS filtering to DAMON, with the series
"mm/damon/core: implement damos filter".
These filters provide users with finer-grained control over DAMOS's
actions. SeongJae has also done some DAMON cleanup work.
- Kairui Song adds a series ("Clean up and fixes for swap").
- Vernon Yang contributed the series "Clean up and refinement for maple
tree".
- Yu Zhao has contributed the "mm: multi-gen LRU: memcg LRU" series. It
adds to MGLRU an LRU of memcgs, to improve the scalability of global
reclaim.
- David Hildenbrand has added some userfaultfd cleanup work in the
series "mm: uffd-wp + change_protection() cleanups".
- Christoph Hellwig has removed the generic_writepages() library
function in the series "remove generic_writepages".
- Baolin Wang has performed some maintenance on the compaction code in
his series "Some small improvements for compaction".
- Sidhartha Kumar is doing some maintenance work on struct page in his
series "Get rid of tail page fields".
- David Hildenbrand contributed some cleanup, bugfixing and
generalization of pte management and of pte debugging in his series
"mm: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE on all architectures with
swap PTEs".
- Mel Gorman and Neil Brown have removed the __GFP_ATOMIC allocation
flag in the series "Discard __GFP_ATOMIC".
- Sergey Senozhatsky has improved zsmalloc's memory utilization with
his series "zsmalloc: make zspage chain size configurable".
- Joey Gouly has added prctl() support for prohibiting the creation of
writeable+executable mappings.
The previous BPF-based approach had shortcomings. See "mm: In-kernel
support for memory-deny-write-execute (MDWE)".
- Waiman Long did some kmemleak cleanup and bugfixing in the series
"mm/kmemleak: Simplify kmemleak_cond_resched() & fix UAF".
- T.J. Alumbaugh has contributed some MGLRU cleanup work in his series
"mm: multi-gen LRU: improve".
- Jiaqi Yan has provided some enhancements to our memory error
statistics reporting, mainly by presenting the statistics on a
per-node basis. See the series "Introduce per NUMA node memory error
statistics".
- Mel Gorman has a second and hopefully final shot at fixing a CPU-hog
regression in compaction via his series "Fix excessive CPU usage
during compaction".
- Christoph Hellwig does some vmalloc maintenance work in the series
"cleanup vfree and vunmap".
- Christoph Hellwig has removed block_device_operations.rw_page() in
ths series "remove ->rw_page".
- We get some maple_tree improvements and cleanups in Liam Howlett's
series "VMA tree type safety and remove __vma_adjust()".
- Suren Baghdasaryan has done some work on the maintainability of our
vm_flags handling in the series "introduce vm_flags modifier
functions".
- Some pagemap cleanup and generalization work in Mike Rapoport's
series "mm, arch: add generic implementation of pfn_valid() for
FLATMEM" and "fixups for generic implementation of pfn_valid()"
- Baoquan He has done some work to make /proc/vmallocinfo and
/proc/kcore better represent the real state of things in his series
"mm/vmalloc.c: allow vread() to read out vm_map_ram areas".
- Jason Gunthorpe rationalized the GUP system's interface to the rest
of the kernel in the series "Simplify the external interface for
GUP".
- SeongJae Park wishes to migrate people from DAMON's debugfs interface
over to its sysfs interface. To support this, we'll temporarily be
printing warnings when people use the debugfs interface. See the
series "mm/damon: deprecate DAMON debugfs interface".
- Andrey Konovalov provided the accurately named "lib/stackdepot: fixes
and clean-ups" series.
- Huang Ying has provided a dramatic reduction in migration's TLB flush
IPI rates with the series "migrate_pages(): batch TLB flushing".
- Arnd Bergmann has some objtool fixups in "objtool warning fixes".
* tag 'mm-stable-2023-02-20-13-37' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm: (505 commits)
include/linux/migrate.h: remove unneeded externs
mm/memory_hotplug: cleanup return value handing in do_migrate_range()
mm/uffd: fix comment in handling pte markers
mm: change to return bool for isolate_movable_page()
mm: hugetlb: change to return bool for isolate_hugetlb()
mm: change to return bool for isolate_lru_page()
mm: change to return bool for folio_isolate_lru()
objtool: add UACCESS exceptions for __tsan_volatile_read/write
kmsan: disable ftrace in kmsan core code
kasan: mark addr_has_metadata __always_inline
mm: memcontrol: rename memcg_kmem_enabled()
sh: initialize max_mapnr
m68k/nommu: add missing definition of ARCH_PFN_OFFSET
mm: percpu: fix incorrect size in pcpu_obj_full_size()
maple_tree: reduce stack usage with gcc-9 and earlier
mm: page_alloc: call panic() when memoryless node allocation fails
mm: multi-gen LRU: avoid futile retries
migrate_pages: move THP/hugetlb migration support check to simplify code
migrate_pages: batch flushing TLB
migrate_pages: share more code between _unmap and _move
...
249 lines
12 KiB
ReStructuredText
249 lines
12 KiB
ReStructuredText
========
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zsmalloc
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========
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This allocator is designed for use with zram. Thus, the allocator is
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supposed to work well under low memory conditions. In particular, it
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never attempts higher order page allocation which is very likely to
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fail under memory pressure. On the other hand, if we just use single
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(0-order) pages, it would suffer from very high fragmentation --
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any object of size PAGE_SIZE/2 or larger would occupy an entire page.
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This was one of the major issues with its predecessor (xvmalloc).
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To overcome these issues, zsmalloc allocates a bunch of 0-order pages
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and links them together using various 'struct page' fields. These linked
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pages act as a single higher-order page i.e. an object can span 0-order
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page boundaries. The code refers to these linked pages as a single entity
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called zspage.
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For simplicity, zsmalloc can only allocate objects of size up to PAGE_SIZE
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since this satisfies the requirements of all its current users (in the
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worst case, page is incompressible and is thus stored "as-is" i.e. in
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uncompressed form). For allocation requests larger than this size, failure
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is returned (see zs_malloc).
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Additionally, zs_malloc() does not return a dereferenceable pointer.
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Instead, it returns an opaque handle (unsigned long) which encodes actual
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location of the allocated object. The reason for this indirection is that
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zsmalloc does not keep zspages permanently mapped since that would cause
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issues on 32-bit systems where the VA region for kernel space mappings
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is very small. So, before using the allocating memory, the object has to
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be mapped using zs_map_object() to get a usable pointer and subsequently
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unmapped using zs_unmap_object().
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stat
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====
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With CONFIG_ZSMALLOC_STAT, we could see zsmalloc internal information via
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``/sys/kernel/debug/zsmalloc/<user name>``. Here is a sample of stat output::
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# cat /sys/kernel/debug/zsmalloc/zram0/classes
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class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage
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...
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...
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9 176 0 1 186 129 8 4
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10 192 1 0 2880 2872 135 3
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11 208 0 1 819 795 42 2
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12 224 0 1 219 159 12 4
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...
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...
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class
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index
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size
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object size zspage stores
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almost_empty
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the number of ZS_ALMOST_EMPTY zspages(see below)
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almost_full
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the number of ZS_ALMOST_FULL zspages(see below)
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obj_allocated
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the number of objects allocated
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obj_used
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the number of objects allocated to the user
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pages_used
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the number of pages allocated for the class
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pages_per_zspage
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the number of 0-order pages to make a zspage
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We assign a zspage to ZS_ALMOST_EMPTY fullness group when n <= N / f, where
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* n = number of allocated objects
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* N = total number of objects zspage can store
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* f = fullness_threshold_frac(ie, 4 at the moment)
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Similarly, we assign zspage to:
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* ZS_ALMOST_FULL when n > N / f
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* ZS_EMPTY when n == 0
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* ZS_FULL when n == N
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Internals
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=========
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zsmalloc has 255 size classes, each of which can hold a number of zspages.
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Each zspage can contain up to ZSMALLOC_CHAIN_SIZE physical (0-order) pages.
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The optimal zspage chain size for each size class is calculated during the
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creation of the zsmalloc pool (see calculate_zspage_chain_size()).
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As an optimization, zsmalloc merges size classes that have similar
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characteristics in terms of the number of pages per zspage and the number
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of objects that each zspage can store.
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For instance, consider the following size classes:::
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class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage freeable
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...
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94 1536 0 0 0 0 0 3 0
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100 1632 0 0 0 0 0 2 0
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...
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Size classes #95-99 are merged with size class #100. This means that when we
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need to store an object of size, say, 1568 bytes, we end up using size class
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#100 instead of size class #96. Size class #100 is meant for objects of size
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1632 bytes, so each object of size 1568 bytes wastes 1632-1568=64 bytes.
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Size class #100 consists of zspages with 2 physical pages each, which can
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hold a total of 5 objects. If we need to store 13 objects of size 1568, we
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end up allocating three zspages, or 6 physical pages.
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However, if we take a closer look at size class #96 (which is meant for
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objects of size 1568 bytes) and trace `calculate_zspage_chain_size()`, we
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find that the most optimal zspage configuration for this class is a chain
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of 5 physical pages:::
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pages per zspage wasted bytes used%
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1 960 76
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2 352 95
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3 1312 89
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4 704 95
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5 96 99
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This means that a class #96 configuration with 5 physical pages can store 13
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objects of size 1568 in a single zspage, using a total of 5 physical pages.
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This is more efficient than the class #100 configuration, which would use 6
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physical pages to store the same number of objects.
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As the zspage chain size for class #96 increases, its key characteristics
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such as pages per-zspage and objects per-zspage also change. This leads to
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dewer class mergers, resulting in a more compact grouping of classes, which
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reduces memory wastage.
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Let's take a closer look at the bottom of `/sys/kernel/debug/zsmalloc/zramX/classes`:::
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class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage freeable
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...
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202 3264 0 0 0 0 0 4 0
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254 4096 0 0 0 0 0 1 0
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...
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Size class #202 stores objects of size 3264 bytes and has a maximum of 4 pages
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per zspage. Any object larger than 3264 bytes is considered huge and belongs
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to size class #254, which stores each object in its own physical page (objects
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in huge classes do not share pages).
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Increasing the size of the chain of zspages also results in a higher watermark
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for the huge size class and fewer huge classes overall. This allows for more
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efficient storage of large objects.
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For zspage chain size of 8, huge class watermark becomes 3632 bytes:::
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class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage freeable
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...
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202 3264 0 0 0 0 0 4 0
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211 3408 0 0 0 0 0 5 0
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217 3504 0 0 0 0 0 6 0
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222 3584 0 0 0 0 0 7 0
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225 3632 0 0 0 0 0 8 0
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254 4096 0 0 0 0 0 1 0
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...
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For zspage chain size of 16, huge class watermark becomes 3840 bytes:::
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class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage freeable
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...
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202 3264 0 0 0 0 0 4 0
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206 3328 0 0 0 0 0 13 0
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207 3344 0 0 0 0 0 9 0
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208 3360 0 0 0 0 0 14 0
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211 3408 0 0 0 0 0 5 0
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212 3424 0 0 0 0 0 16 0
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214 3456 0 0 0 0 0 11 0
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217 3504 0 0 0 0 0 6 0
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219 3536 0 0 0 0 0 13 0
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222 3584 0 0 0 0 0 7 0
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223 3600 0 0 0 0 0 15 0
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225 3632 0 0 0 0 0 8 0
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228 3680 0 0 0 0 0 9 0
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230 3712 0 0 0 0 0 10 0
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232 3744 0 0 0 0 0 11 0
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234 3776 0 0 0 0 0 12 0
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235 3792 0 0 0 0 0 13 0
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236 3808 0 0 0 0 0 14 0
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238 3840 0 0 0 0 0 15 0
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254 4096 0 0 0 0 0 1 0
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...
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Overall the combined zspage chain size effect on zsmalloc pool configuration:::
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pages per zspage number of size classes (clusters) huge size class watermark
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4 69 3264
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5 86 3408
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6 93 3504
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7 112 3584
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8 123 3632
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9 140 3680
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10 143 3712
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11 159 3744
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12 164 3776
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13 180 3792
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14 183 3808
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15 188 3840
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16 191 3840
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A synthetic test
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----------------
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zram as a build artifacts storage (Linux kernel compilation).
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* `CONFIG_ZSMALLOC_CHAIN_SIZE=4`
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zsmalloc classes stats:::
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class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage freeable
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...
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Total 13 51 413836 412973 159955 3
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zram mm_stat:::
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1691783168 628083717 655175680 0 655175680 60 0 34048 34049
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* `CONFIG_ZSMALLOC_CHAIN_SIZE=8`
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zsmalloc classes stats:::
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class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage freeable
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...
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Total 18 87 414852 412978 156666 0
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zram mm_stat:::
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1691803648 627793930 641703936 0 641703936 60 0 33591 33591
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Using larger zspage chains may result in using fewer physical pages, as seen
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in the example where the number of physical pages used decreased from 159955
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to 156666, at the same time maximum zsmalloc pool memory usage went down from
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655175680 to 641703936 bytes.
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However, this advantage may be offset by the potential for increased system
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memory pressure (as some zspages have larger chain sizes) in cases where there
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is heavy internal fragmentation and zspool compaction is unable to relocate
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objects and release zspages. In these cases, it is recommended to decrease
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the limit on the size of the zspage chains (as specified by the
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CONFIG_ZSMALLOC_CHAIN_SIZE option).
|