+ mm-huge_memory-transfer-the-pmd-dirty-bit-to-the-folio-on-zap.patch added to mm-hotfixes-unstable branch

The patch titled
     Subject: mm/huge_memory: transfer the pmd dirty bit to the folio on zap
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-huge_memory-transfer-the-pmd-dirty-bit-to-the-folio-on-zap.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-transfer-the-pmd-dirty-bit-to-the-folio-on-zap.patch

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

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------------------------------------------------------
From: Usama Arif <[email protected]>
Subject: mm/huge_memory: transfer the pmd dirty bit to the folio on zap
Date: Wed, 19 Aug 2026 03:12:22 -0700

zap_huge_pmd_folio() propagates the pmd young bit to the folio for the
file case, but not the dirty bit.  The pte path does propagate it, in
zap_present_folio_ptes() and so does the pmd split path, in
__split_huge_pmd_locked().

For most file mappings the omission is harmless, because writing to a
shared file mapping goes through page_mkwrite(), which dirties the folio. 
tmpfs is different: it has no page_mkwrite(), and vma_wants_writenotify()
is false for it, so a *read* fault on a MAP_SHARED tmpfs mapping installs
a writable pmd via do_read_fault().  do_read_fault() does not call
fault_dirty_shared_page(), so subsequent stores through that mapping set
only the hardware dirty bit in the pmd and never call folio_mark_dirty(). 
A shmem folio allocated by a fault is marked uptodate but not dirty (see
the clear: block in shmem_get_folio_gfp()), so PG_dirty is never set at
all.

Unmapping such a folio - munmap(), or exit_mmap() when the process dies -
then loses the only record that it was written, because zap_huge_pmd()
drops the pmd without transferring the dirty bit.  Reclaim afterwards sees
a clean shmem folio: the whole swap-out block in shrink_folio_list() is
inside "if (folio_test_dirty(folio))", so pageout() is skipped and the
folio falls into __remove_mapping().  There, folio_is_file_lru() is false
for a swapbacked folio, so no shadow entry is created and
__filemap_remove_folio(folio, NULL) simply empties the i_pages slot.  The
data is freed without ever being written to swap, and the next fault on
that index returns a freshly zeroed folio.

This is silent data loss for any process that keeps state in a MAP_SHARED
tmpfs segment across an unmap - for example a cache handed from one
process generation to the next through /dev/shm.  It requires the folio to
be PMD-mapped, so it only shows up once shmem THP is enabled (which is
what we did in Meta fleet and started noticing crashes); with THP off the
pte path transfers the dirty bit correctly.  It also only becomes visible
when swap is enabled, because with no swap device shmem folios (which are
on the anon LRU) are not scanned by reclaim at all, so the clean folio is
never dropped.

Reproduced on x86_64 with a tmpfs mounted huge=within_size: read-fault a
2MB-backed region, write a known pattern through the resulting mapping,
munmap, force reclaim of the cgroup, then re-map and read back.  Without
this patch the region reads back as zeros and vmstat shows zswpout 0 - the
data was discarded rather than swapped.  With this patch the region reads
back correctly and the pages are swapped out as expected.  With
huge=never, or when the first touch is a write, the test passes either
way.

Link: https://lore.kernel.org/[email protected]
Fixes: b5072380eb61 ("thp: support file pages in zap_huge_pmd()")
Signed-off-by: Usama Arif <[email protected]>
Acked-by: David Hildenbrand (Arm) <[email protected]>
Reviewed-by: Kiryl Shutsemau <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Tested-by: Lance Yang <[email protected]>
Reviewed-by: Zi Yan <[email protected]>
Reviewed-by: Lorenzo Stoakes (ARM) <[email protected]>
Reviewed-by: Baolin Wang <[email protected]>
Cc: Barry Song <[email protected]>
Cc: Dev Jain <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Cc: Nhat Pham <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
---

 mm/huge_memory.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/mm/huge_memory.c~mm-huge_memory-transfer-the-pmd-dirty-bit-to-the-folio-on-zap
+++ a/mm/huge_memory.c
@@ -2414,6 +2414,8 @@ static void zap_huge_pmd_folio(struct mm
 		add_mm_counter(mm, mm_counter_file(folio),
 			       -HPAGE_PMD_NR);
 
+		if (is_present && pmd_dirty(pmdval))
+			folio_mark_dirty(folio);
 		if (is_present && pmd_young(pmdval) &&
 		    likely(vma_has_recency(vma)))
 			folio_mark_accessed(folio);
_

Patches currently in -mm which might be from [email protected] are

mm-huge_memory-transfer-the-pmd-dirty-bit-to-the-folio-on-zap.patch
mm-vmstat-mm-memcontrol-add-_monotonic-vmstat-readers.patch
mm-vmscan-add-pgrotate_anon-and-pgrotate_file-vmstat-counters.patch
mm-vmscan-reduce-lru_lock-contention-via-vmstat-derived-scan-balance-cost.patch
mm-memcontrol-avoid-false-sharing-between-vmstats-and-events.patch