+ mm-huge_memory-fix-early-failure-try_to_migrate-when-split-huge-pmd-for-shared-thp.patch added to mm-new branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/huge_memory: fix early failure try_to_migrate() when split huge pmd for shared thp
has been added to the -mm mm-new branch.  Its filename is
     mm-huge_memory-fix-early-failure-try_to_migrate-when-split-huge-pmd-for-shared-thp.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-huge_memory-fix-early-failure-try_to_migrate-when-split-huge-pmd-for-shared-thp.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

The mm-new branch of mm.git is not included in linux-next

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------------------------------------------------------
From: Wei Yang <richard.weiyang@gmail.com>
Subject: mm/huge_memory: fix early failure try_to_migrate() when split huge pmd for shared thp
Date: Wed, 4 Feb 2026 00:42:19 +0000

Commit 60fbb14396d5 ("mm/huge_memory: adjust try_to_migrate_one() and
split_huge_pmd_locked()") return false unconditionally after
split_huge_pmd_locked() which may fail early during try_to_migrate() for
shared thp.  This will lead to unexpected folio split failure.

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.

The reason is the above commit return false after split pmd
unconditionally in the first process and break try_to_migrate().

The tricky thing in above reproduce method is current debugfs interface
leverage function split_huge_pages_pid(), which will iterate the whole pmd
range and do folio split on each base page address.  This means it will
try 512 times, and each time split one pmd from pmd mapped to pte mapped
thp.  If there are less than 512 shared mapped process, the folio is still
split successfully at last.  But in real world, we usually try it for
once.

This patch fixes this by restart page_vma_mapped_walk() after
split_huge_pmd_locked().  Because split_huge_pmd_locked() may fall back to
(freeze = false) if folio_try_share_anon_rmap_pmd() fails and the PMD is
just split instead of split to migration entry.  Restart
page_vma_mapped_walk() and let try_to_migrate_one() try on each PTE again
and fail try_to_migrate() early if it fails.

Link: https://lkml.kernel.org/r/20260204004219.6524-1-richard.weiyang@gmail.com
Fixes: 60fbb14396d5 ("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>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Harry Yoo <harry.yoo@oracle.com>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Zi Yan <ziy@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/rmap.c |   11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

--- a/mm/rmap.c~mm-huge_memory-fix-early-failure-try_to_migrate-when-split-huge-pmd-for-shared-thp
+++ a/mm/rmap.c
@@ -2446,11 +2446,16 @@ static bool try_to_migrate_one(struct fo
 			__maybe_unused pmd_t pmdval;
 
 			if (flags & TTU_SPLIT_HUGE_PMD) {
+				/*
+				 * After split_huge_pmd_locked(), restart the
+				 * walk to detect PageAnonExclusive handling
+				 * failure in __split_huge_pmd_locked().
+				 */
 				split_huge_pmd_locked(vma, pvmw.address,
 						      pvmw.pmd, true);
-				ret = false;
-				page_vma_mapped_walk_done(&pvmw);
-				break;
+				flags &= ~TTU_SPLIT_HUGE_PMD;
+				page_vma_mapped_walk_restart(&pvmw);
+				continue;
 			}
 #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
 			pmdval = pmdp_get(pvmw.pmd);
_

Patches currently in -mm which might be from richard.weiyang@gmail.com are

mm-huge_memory-fix-early-failure-try_to_migrate-when-split-huge-pmd-for-shared-thp.patch