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Wei Yang 6e096db800 mm/huge_memory: fix early failure try_to_migrate() when split huge pmd for shared THP 
commit 939080834f upstream.

Commit 60fbb14396 ("mm/huge_memory: adjust try_to_migrate_one() and
split_huge_pmd_locked()") return false unconditionally after
split_huge_pmd_locked().  This may fail try_to_migrate() early when
TTU_SPLIT_HUGE_PMD is specified.

The reason is the above commit adjusted try_to_migrate_one() to, when a
PMD-mapped THP entry is found, and TTU_SPLIT_HUGE_PMD is specified (for
example, via unmap_folio()), return false unconditionally.  This breaks
the rmap walk and fail try_to_migrate() early, if this PMD-mapped THP is
mapped in multiple processes.

The user sensible impact of this bug could be:

  * On memory pressure, shrink_folio_list() may split partially mapped
    folio with split_folio_to_list(). Then free unmapped pages without IO.
    If failed, it may not be reclaimed.
  * On memory failure, memory_failure() would call try_to_split_thp_page()
    to split folio contains the bad page. If succeed, the PG_has_hwpoisoned
    bit is only set in the after-split folio contains @split_at. By doing
    so, we limit bad memory. If failed to split, the whole folios is not
    usable.

One way to reproduce:

    Create an anonymous THP range and fork 512 children, so we have a
    THP shared mapped in 513 processes. Then trigger folio split with
    /sys/kernel/debug/split_huge_pages debugfs to split the THP folio to
    order 0.

Without the above commit, we can successfully split to order 0.  With the
above commit, the folio is still a large folio.

And currently there are two core users of TTU_SPLIT_HUGE_PMD:

  * try_to_unmap_one()
  * try_to_migrate_one()

try_to_unmap_one() would restart the rmap walk, so only
try_to_migrate_one() is affected.

We can't simply revert commit 60fbb14396 ("mm/huge_memory: adjust
try_to_migrate_one() and split_huge_pmd_locked()"), since it removed some
duplicated check covered by page_vma_mapped_walk().

This patch fixes this by restart page_vma_mapped_walk() after
split_huge_pmd_locked().  Since we cannot simply return "true" to fix the
problem, as that would affect another case:

    When invoking folio_try_share_anon_rmap_pmd() from
    split_huge_pmd_locked(), the latter can fail and leave a large folio
    mapped through PTEs, in which case we ought to return true from
    try_to_migrate_one(). This might result in unnecessary walking of the
    rmap but is relatively harmless.

Link: https://lkml.kernel.org/r/20260305015006.27343-1-richard.weiyang@gmail.com
Fixes: 60fbb14396 ("mm/huge_memory: adjust try_to_migrate_one() and split_huge_pmd_locked()")
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Tested-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Gavin Guo <gavinguo@igalia.com>
Acked-by: David Hildenbrand (arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
History 2026-03-25 11:10:30 +01:00
arch KVM: SVM: Set/clear CR8 write interception when AVIC is (de)activated 2026-03-19 16:08:50 +01:00
block block: use trylock to avoid lockdep circular dependency in sysfs 2026-03-12 07:09:59 -04:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto crypto: authencesn - reject too-short AAD (assoclen<8) to match ESP/ESN spec 2026-01-30 10:32:12 +01:00
Documentation KVM: x86: Introduce KVM_X86_QUIRK_VMCS12_ALLOW_FREEZE_IN_SMM 2026-03-19 16:08:24 +01:00
drivers nvdimm/bus: Fix potential use after free in asynchronous initialization 2026-03-25 11:10:30 +01:00
fs nfsd: fix heap overflow in NFSv4.0 LOCK replay cache 2026-03-25 11:10:30 +01:00
include io_uring: ensure ctx->rings is stable for task work flags manipulation 2026-03-19 16:08:51 +01:00
init printk changes for 6.18 2025-10-04 11:13:11 -07:00
io_uring io_uring/eventfd: use ctx->rings_rcu for flags checking 2026-03-19 16:08:51 +01:00
ipc ipc: don't audit capability check in ipc_permissions() 2026-02-26 14:59:19 -08:00
kernel crash_dump: don't log dm-crypt key bytes in read_key_from_user_keying 2026-03-25 11:10:30 +01:00
lib lib/crypto: tests: Depend on library options rather than selecting them 2026-03-19 16:08:50 +01:00
LICENSES LICENSES: Replace the obsolete address of the FSF in the GFDL-1.2 2025-07-24 11:15:39 +02:00
mm mm/huge_memory: fix early failure try_to_migrate() when split huge pmd for shared THP 2026-03-25 11:10:30 +01:00
net sunrpc: fix cache_request leak in cache_release 2026-03-25 11:10:30 +01:00
rust rust: str: make NullTerminatedFormatter public 2026-03-19 16:08:24 +01:00
samples samples/ftrace: Adjust LoongArch register restore order in direct calls 2026-01-08 10:17:11 +01:00
scripts kbuild: rpm-pkg: Disable automatic requires for manual debuginfo package 2026-03-04 07:21:32 -05:00
security apparmor: fix race between freeing data and fs accessing it 2026-03-13 17:23:30 +01:00
sound ALSA: usb-audio: Improve Focusrite sample rate filtering 2026-03-19 16:08:31 +01:00
tools selftests/hid: fix compilation when bpf_wq and hid_device are not exported 2026-03-25 11:10:30 +01:00
usr gen_init_cpio: Ignore fsync() returning EINVAL on pipes 2025-10-07 09:53:05 -07:00
virt KVM: Don't clobber irqfd routing type when deassigning irqfd 2026-02-11 13:41:44 +01:00
.clang-format memblock: drop for_each_free_mem_pfn_range_in_zone_from() 2025-09-14 08:49:03 +03:00
.clippy.toml rust: clean Rust 1.88.0's warning about clippy::disallowed_macros configuration 2025-05-07 00:11:47 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore MAINTAINERS: remove Alyssa Rosenzweig 2025-09-18 21:17:31 +02:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore .gitignore: ignore compile_commands.json globally 2025-08-12 15:53:55 -07:00
.mailmap 8 hotfixes. 4 are cc:stable, 7 are against mm/. 2025-11-26 12:38:05 -08:00
.pylintrc tools: docs: parse-headers.py: move it from sphinx dir 2025-08-29 15:54:42 -06:00
.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 MAINTAINERS: mark ISDN subsystem as orphan 2025-10-27 17:49:45 -07:00
Kbuild sched: Make migrate_{en,dis}able() inline 2025-09-25 09:57:16 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS soc: fixes for 6.18, part 4 2025-11-28 09:57:31 -08:00
Makefile Linux 6.18.19 2026-03-19 16:08:51 +01:00
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.