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mm/huge_memory: fix folio isn't locked in softleaf_to_folio()

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On arm64 server, we found folio that get from migration entry isn't locked
in softleaf_to_folio().  This issue triggers when mTHP splitting and
zap_nonpresent_ptes() races, and the root cause is lack of memory barrier
in softleaf_to_folio().  The race is as follows:

	CPU0                                             CPU1

deferred_split_scan()                              zap_nonpresent_ptes()
  lock folio
  split_folio()
    unmap_folio()
      change ptes to migration entries
    __split_folio_to_order()                         softleaf_to_folio()
      set flags(including PG_locked) for tail pages    folio = pfn_folio(softleaf_to_pfn(entry))
      smp_wmb()                                        VM_WARN_ON_ONCE(!folio_test_locked(folio))
      prep_compound_page() for tail pages

In __split_folio_to_order(), smp_wmb() guarantees page flags of tail pages
are visible before the tail page becomes non-compound.  smp_wmb() should
be paired with smp_rmb() in softleaf_to_folio(), which is missed.  As a
result, if zap_nonpresent_ptes() accesses migration entry that stores tail
pfn, softleaf_to_folio() may see the updated compound_head of tail page
before page->flags.

This issue will trigger VM_WARN_ON_ONCE() in pfn_swap_entry_folio()
because of the race between folio split and zap_nonpresent_ptes()
leading to a folio incorrectly undergoing modification without a folio
lock being held.

This is a BUG_ON() before commit 93976a2034 ("mm: eliminate further
swapops predicates"), which in merged in v6.19-rc1.

To fix it, add missing smp_rmb() if the softleaf entry is migration entry
in softleaf_to_folio() and softleaf_to_page().

[tujinjiang@huawei.com: update function name and comments]
  Link: https://lkml.kernel.org/r/20260321075214.3305564-1-tujinjiang@huawei.com
Link: https://lkml.kernel.org/r/20260319012541.4158561-1-tujinjiang@huawei.com
Fixes: e9b61f1985 ("thp: reintroduce split_huge_page()")
Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Barry Song <baohua@kernel.org>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nanyong Sun <sunnanyong@huawei.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Jinjiang Tu 2026-03-19 09:25:41 +08:00 committed by Andrew Morton
parent 224f129261
commit 4ff07459db
1 changed files with 21 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

32
include/linux/leafops.h
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@ -363,6 +363,23 @@ static inline unsigned long softleaf_to_pfn(softleaf_t entry)
return swp_offset(entry) & SWP_PFN_MASK;
}
static inline void softleaf_migration_sync(softleaf_t entry,
struct folio *folio)
{
/*
* Ensure we do not race with split, which might alter tail pages into new
* folios and thus result in observing an unlocked folio.
* This matches the write barrier in __split_folio_to_order().
*/
smp_rmb();
/*
* Any use of migration entries may only occur while the
* corresponding page is locked
*/
VM_WARN_ON_ONCE(!folio_test_locked(folio));
}
/**
* softleaf_to_page() - Obtains struct page for PFN encoded within leaf entry.
* @entry: Leaf entry, softleaf_has_pfn(@entry) must return true.
@ -374,11 +391,8 @@ static inline struct page *softleaf_to_page(softleaf_t entry)
struct page *page = pfn_to_page(softleaf_to_pfn(entry));
VM_WARN_ON_ONCE(!softleaf_has_pfn(entry));
/*
* Any use of migration entries may only occur while the
* corresponding page is locked
*/
VM_WARN_ON_ONCE(softleaf_is_migration(entry) && !PageLocked(page));
if (softleaf_is_migration(entry))
softleaf_migration_sync(entry, page_folio(page));
return page;
}
@ -394,12 +408,8 @@ static inline struct folio *softleaf_to_folio(softleaf_t entry)
struct folio *folio = pfn_folio(softleaf_to_pfn(entry));
VM_WARN_ON_ONCE(!softleaf_has_pfn(entry));
/*
* Any use of migration entries may only occur while the
* corresponding folio is locked.
*/
VM_WARN_ON_ONCE(softleaf_is_migration(entry) &&
!folio_test_locked(folio));
if (softleaf_is_migration(entry))
softleaf_migration_sync(entry, folio);
return folio;
}