Re: [PATCH] mm/khugepaged: don't map folios that are not uptodate in MADV_COLLAPSE

[Kiryl Shutsemau]

On Thu, Nov 05, 2026 at 07:16:44PM +0100, David Hildenbrand (Arm) wrote:
> madvise(MADV_COLLAPSE) can map PMD-sized pagecache folios that are not
> uptodate. For example, shmem fallocate() can put such folios into the
> page cache.
> 
> collapse_file() already refuses to proceed with folios that are not
> uptodate. Let's just do the same in try_collapse_pte_mapped_thp() and skip
> such folios, leaving handling to the page faults.
> 
> Note that collapse_file() effectively clears shmem folios through
> shmem_get_folio(); let's keep this fix simple and simply let the page
> fault path deal with that.
> 
> Fixes: 34488399fa08 ("mm/madvise: add file and shmem support to MADV_COLLAPSE")
> Reported-by: Chris Mason <mason@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

Reviewed-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>

Separately from this fix: nothing on the mapping side catches this.
Every caller checks uptodate on its own (next_uptodate_folio(),
filemap_fault(), shmem_get_folio()), and khugepaged was the one that
went straight from filemap_lock_folio() to do_set_pmd().

Should do_set_pmd() and set_pte_range() assert it? Something like

        VM_WARN_ON_ONCE_FOLIO(folio->mapping && !folio_test_anon(folio) &&
                              !folio_test_uptodate(folio), folio);

The ->mapping test is for set_pte_range(): finish_fault() also maps
driver pages that never get PG_uptodate.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov