Re: [PATCH] mm/huge_memory: transfer the pmd dirty bit to the folio on zap

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Wed, Aug 19, 2026 at 03:12:22AM -0700, Usama Arif wrote:
> 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.

Good lord.

>
> 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.
>
> Fixes: 800d8c63b2e9 ("shmem: add huge pages support")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Usama Arif <usama.arif@linux.dev>

Thanks for this! LGTM so:

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  mm/huge_memory.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index ced400f72d43a..afbb5974bd225 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2449,6 +2449,8 @@ static void zap_huge_pmd_folio(struct mm_struct *mm, struct vm_area_struct *vma,
>  		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);
> --
> 2.53.0-Meta
>

--
Cheers, Lorenzo