Re: [PATCH 1/2] mm: memfd_luo: always make all folios uptodate

[Mike Rapoport <rppt@kernel.org>]

On Mon, Feb 23, 2026 at 06:39:28PM +0100, Pratyush Yadav wrote:
> From: "Pratyush Yadav (Google)" <pratyush@kernel.org>
> 
> When a folio is added to a shmem file via fallocate, it is not zeroed on
> allocation. This is done as a performance optimization since it is
> possible the folio will never end up being used at all. When the folio
> is used, shmem checks for the uptodate flag, and if absent, zeroes the
> folio (and sets the flag) before returning to user.
> 
> With LUO, the flags of each folio are saved at preserve time. It is
> possible to have a memfd with some folios fallocated but not uptodate.
> For those, the uptodate flag doesn't get saved. The folios might later
> end up being used and become uptodate. They would get passed to the next
> kernel via KHO correctly since they did get preserved. But they won't
> have the MEMFD_LUO_FOLIO_UPTODATE flag.
> 
> This means that when the memfd is retrieved, the folios will be added to
> the shmem file without the uptodate flag. They will be zeroed before
> first use, losing the data in those folios.
> 
> Since we take a big performance hit in allocating, zeroing, and pinning
> all folios at prepare time anyway, take some more and zero all
> non-uptodate ones too.
> 
> Later when there is a stronger need to make prepare faster, this can be
> optimized.
> 
> To avoid racing with another uptodate operation, take the folio lock.
> 
> Fixes: b3749f174d68 ("mm: memfd_luo: allow preserving memfd")
> Cc: stable@vger.kernel.org
> Signed-off-by: Pratyush Yadav (Google) <pratyush@kernel.org>

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
>  mm/memfd_luo.c | 25 +++++++++++++++++++++++--
>  1 file changed, 23 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memfd_luo.c b/mm/memfd_luo.c
> index a34fccc23b6a..ccbf1337f650 100644
> --- a/mm/memfd_luo.c
> +++ b/mm/memfd_luo.c
> @@ -152,10 +152,31 @@ static int memfd_luo_preserve_folios(struct file *file,
>  		if (err)
>  			goto err_unpreserve;
>  
> +		folio_lock(folio);
> +
>  		if (folio_test_dirty(folio))
>  			flags |= MEMFD_LUO_FOLIO_DIRTY;
> -		if (folio_test_uptodate(folio))
> -			flags |= MEMFD_LUO_FOLIO_UPTODATE;
> +
> +		/*
> +		 * If the folio is not uptodate, it was fallocated but never
> +		 * used. Saving this flag at prepare() doesn't work since it
> +		 * might change later when someone uses the folio.
> +		 *
> +		 * Since we have taken the performance penalty of allocating,
> +		 * zeroing, and pinning all the folios in the holes, take a bit
> +		 * more and zero all non-uptodate folios too.
> +		 *
> +		 * NOTE: For someone looking to improve preserve performance,
> +		 * this is a good place to look.

I'd add a larger comment above memfd_luo_preserve_folios() that says that
it allocates, pins etc and fold the last two paragraphs of this comment
there.

> +		 */
> +		if (!folio_test_uptodate(folio)) {
> +			folio_zero_range(folio, 0, folio_size(folio));
> +			flush_dcache_folio(folio);
> +			folio_mark_uptodate(folio);
> +		}
> +		flags |= MEMFD_LUO_FOLIO_UPTODATE;
> +
> +		folio_unlock(folio);
>  
>  		pfolio->pfn = folio_pfn(folio);
>  		pfolio->flags = flags;
> -- 
> 2.53.0.371.g1d285c8824-goog
> 

-- 
Sincerely yours,
Mike.