Re: [PATCH 17/46] mm/khugepaged: preserve dirty bits when collapsing PTE-mapped THPs

[Pedro Falcato]

On Tue, Oct 27, 2026 at 12:17:37AM +0100, 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 ]
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

Reviewed-by: Pedro Falcato <pfalcato@xxxxxxx>

-- 
Pedro