Re: [PATCH v3 17/47] mm/khugepaged: preserve dirty bits when collapsing PTE-mapped THPs

[Baolin Wang]

On 10/31/26 5:39 PM, David Hildenbrand (Arm) wrote:
> From: Chris Mason <mason@xxxxxxxxxx>
>
> try_collapse_pte_mapped_thp() clears PTEs without transferring their dirty
> bits to the folio. A shmem read fault can map a clean folio writable, so
> a later store only dirties the PTE. Losing that dirty bit allows reclaim
> to discard modified data.
>
> The assumption that regular files are read-only is also obsolete since
> commit 1b6444331bebb ("mm/khugepaged: enable clean pagecache folio collapse
> for writable files"). In any case, transferring PTE dirty bits to the folio
> when removing mappings (and while the folio is still mapped) is the right
> thing to do.
>
> Use get_and_clear_ptes() to collect the dirty bits and mark the folio
> dirty before unmapping it. Update the comment.
>
> This was reproduced with shmem using MADV_COLLAPSE followed by
> MADV_PAGEOUT.
>
> Fix generated by an LLM and sent by Chris Mason in private.
>
> Fixes: 27e1f8273113 ("khugepaged: enable collapse pmd for pte-mapped THP")
> Fixes: 1b6444331bebb ("mm/khugepaged: enable clean pagecache folio collapse for writable files")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Chris Mason <mason@xxxxxxxxxx>
> [ david: adjust comment; simplify description; cover file-thp as well ]
> Reviewed-by: Pedro Falcato <pfalcato@xxxxxxx>
> Reviewed-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
> Tested-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
> Tested-by: Muhammad Usama Anjum <usama.anjum@xxxxxxx>
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> ---

LGTM.
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>