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

[Hugh Dickins <hughd@google.com>]

On Thu, 5 Nov 2026, 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@kernel.org>
> Cc: stable@vger.kernel.org
> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>

Acked-by: Hugh Dickins <hughd@google.com>

Valuable catch, thanks. I was worried that "try_collapse_pte_mapped_thp"
implies there's already a PTE mapping the non-uptodate folio, which
would already be wrong. But we've been a little too free with the
SCAN_PTE_MAPPED_HUGEPAGE result code, so there's no such implication
(and even if there had been a good PTE, I doubt there's enough locking
to stop racing fallocates putting a non-uptodate folio there meanwhile).

I was going to quibble with the Fixes, thinking the problem was there
earlier: but no, I agree with you, that's when install_pmd came in.

> ---
> Once reviewed, I will pick this up in a timely manner.
> ---
>  mm/khugepaged.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 8d9f05a39afb3..db135b3c44ef1 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1900,6 +1900,12 @@ static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsign
>  		goto drop_folio;
>  	}
>  
> +	/* Leave any uninitialized folios to the page fault path. */
> +	if (!folio_test_uptodate(folio)) {
> +		result = SCAN_FAIL;
> +		goto drop_folio;
> +	}
> +
>  	result = find_pmd_or_thp_or_none(mm, haddr, &pmd);
>  	switch (result) {
>  	case SCAN_SUCCEED:
> 
> ---
> 
> base-commit: f44fd601f72e09e44c667b9234c9cd469856679c
> 
> change-id: 20261105-set_huge_pmd_uptodate-9b9820307f30
> 
> --
> 
> Cheers,
> 
> David