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Barry Song faeb2831b5 mm: set folio swapbacked iff folios are dirty in try_to_unmap_one 
Patch series "mm: batched unmap lazyfree large folios during reclamation",
v4.

Commit 735ecdfaf4 ("mm/vmscan: avoid split lazyfree THP during
shrink_folio_list()") prevents the splitting of MADV_FREE'd THP in
madvise.c.  

However, those folios are still added to the deferred_split list in
try_to_unmap_one() because we are unmapping PTEs and removing rmap entries
one by one.

Firstly, this has rendered the following counter somewhat confusing,
/sys/kernel/mm/transparent_hugepage/hugepages-size/stats/split_deferred
The split_deferred counter was originally designed to track operations
such as partial unmap or madvise of large folios.  However, in practice,
most split_deferred cases arise from memory reclamation of aligned
lazyfree mTHPs as observed by Tangquan.  This discrepancy has made the
split_deferred counter highly misleading.

Secondly, this approach is slow because it requires iterating through each
PTE and removing the rmap one by one for a large folio.  In fact, all PTEs
of a pte-mapped large folio should be unmapped at once, and the entire
folio should be removed from the rmap as a whole.

Thirdly, it also increases the risk of a race condition where lazyfree
folios are incorrectly set back to swapbacked, as a speculative folio_get
may occur in the shrinker's callback.

deferred_split_scan() might call folio_try_get(folio) since we have added
the folio to split_deferred list while removing rmap for the 1st subpage,
and while we are scanning the 2nd to nr_pages PTEs of this folio in
try_to_unmap_one(), the entire mTHP could be transitioned back to
swap-backed because the reference count is incremented, which can make
"ref_count == 1 + map_count" within try_to_unmap_one() false.

   /*
    * The only page refs must be one from isolation
    * plus the rmap(s) (dropped by discard:).
    */
   if (ref_count == 1 + map_count &&
       (!folio_test_dirty(folio) ||
        ...
        (vma->vm_flags & VM_DROPPABLE))) {
           dec_mm_counter(mm, MM_ANONPAGES);
           goto discard;
   }

This patchset resolves the issue by marking only genuinely dirty folios as
swap-backed, as suggested by David, and transitioning to batched unmapping
of entire folios in try_to_unmap_one().  Consequently, the deferred_split
count drops to zero, and memory reclamation performance improves
significantly — reclaiming 64KiB lazyfree large folios is now 2.5x
faster(The specific data is embedded in the changelog of patch 3/4).

By the way, while the patchset is primarily aimed at PTE-mapped large
folios, Baolin and Lance also found that try_to_unmap_one() handles
lazyfree redirtied PMD-mapped large folios inefficiently — it splits the
PMD into PTEs and iterates over them.  This patchset removes the
unnecessary splitting, enabling us to skip redirtied PMD-mapped large
folios 3.5X faster during memory reclamation.  (The specific data can be
found in the changelog of patch 4/4).


This patch (of 4):

The refcount may be temporarily or long-term increased, but this does not
change the fundamental nature of the folio already being lazy- freed. 
Therefore, we only reset 'swapbacked' when we are certain the folio is
dirty and not droppable.

Link: https://lkml.kernel.org/r/20250214093015.51024-1-21cnbao@gmail.com
Link: https://lkml.kernel.org/r/20250214093015.51024-2-21cnbao@gmail.com
Fixes: 6c8e2a2569 ("mm: fix race between MADV_FREE reclaim and blkdev direct IO read")
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Lance Yang <ioworker0@gmail.com>
Cc: Mauricio Faria de Oliveira <mfo@canonical.com>
Cc: Chis Li <chrisl@kernel.org> (Google)
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Kairui Song <kasong@tencent.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Tangquan Zheng <zhengtangquan@oppo.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Gavin Shan <gshan@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Shaoqin Huang <shahuang@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will@kernel.org>
Cc: Yicong Yang <yangyicong@hisilicon.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
2025-03-16 22:06:16 -07:00
arch mm: zbud: remove zbud 2025-03-16 22:06:01 -07:00
block block-6.14-20250306 2025-03-07 11:12:33 -10:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto treewide: const qualify ctl_tables where applicable 2025-01-28 13:48:37 +01:00
Documentation Docs/admin-guide/mm/damon/usage: document hugepage_size filter type 2025-03-16 22:06:13 -07:00
drivers fb_defio: do not use deprecated page->mapping, index fields 2025-03-16 22:06:11 -07:00
fs percpu: use TYPEOF_UNQUAL() in variable declarations 2025-03-16 22:05:53 -07:00
include mm/mmu_gather: clean up the stale code comment 2025-03-16 22:06:14 -07:00
init Kbuild updates for v6.14 2025-01-31 12:07:07 -08:00
io_uring io_uring-6.14-20250306 2025-03-07 11:09:33 -10:00
ipc treewide: const qualify ctl_tables where applicable 2025-01-28 13:48:37 +01:00
kernel kernel/events/uprobes: handle device-exclusive entries correctly in __replace_page() 2025-03-16 22:05:58 -07:00
lib maple_tree: remove a BUG_ON() in mas_alloc_nodes() 2025-03-16 22:06:15 -07:00
LICENSES LICENSES: add 0BSD license text 2024-09-01 20:43:24 -07:00
mm mm: set folio swapbacked iff folios are dirty in try_to_unmap_one 2025-03-16 22:06:16 -07:00
net percpu: use TYPEOF_UNQUAL() in variable declarations 2025-03-16 22:05:53 -07:00
rust Driver core api addition for 6.14-rc3 2025-02-16 12:54:42 -08:00
samples samples: kmemleak: print the raw pointers for debugging purposes 2025-03-16 22:06:08 -07:00
scripts kbuild: install-extmod-build: Fix build when specifying KBUILD_OUTPUT 2025-03-06 20:32:30 +09:00
security Landlock fix for v6.14-rc5 2025-02-26 11:55:44 -08:00
sound ALSA: hda/realtek: Add support for ASUS Zenbook UM3406KA Laptops using CS35L41 HDA 2025-03-05 19:52:56 +01:00
tools tools/selftests: add file/shmem-backed mapping guard region tests 2025-03-16 22:06:15 -07:00
usr kbuild: hdrcheck: fix cross build with clang 2025-03-05 04:06:45 +09:00
virt KVM: remove kvm_arch_post_init_vm 2025-02-04 11:27:45 -05:00
.clang-format clang-format: Update with v6.11-rc1's for_each macro list 2024-08-02 13:20:31 +02:00
.clippy.toml rust: give Clippy the minimum supported Rust version 2025-01-10 00:17:25 +01:00
.cocciconfig
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore MAINTAINERS: Retire Ralf Baechle 2024-11-12 15:48:59 +01:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore rust: use host dylib naming convention to support macOS 2025-01-10 01:01:24 +01:00
.mailmap 33 hotfixes. 24 are cc:stable and the remainder address post-6.13 issues 2025-03-08 14:34:06 -10:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS mm: zbud: remove zbud 2025-03-16 22:06:01 -07:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS mm: zbud: remove zbud 2025-03-16 22:06:01 -07:00
Makefile Linux 6.14-rc6 2025-03-09 13:45:25 -10:00
README README: Fix spelling 2024-03-18 03:36:32 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.