Re: [PATCH v8 1/7] NFSD: cut a misaligned direct-mode WRITE at the mapping's smallest folio

["Chuck Lever" <cel@kernel.org>]

On Thu, Oct 22, 2026, at 10:45 PM, Mike Snitzer wrote:
> nfsd_write_dio_iters_init() cuts a misaligned end of a direct-mode
> WRITE at a multiple of the larger of offset_align and PAGE_SIZE, so
> that the direct middle, which invalidates the page cache over its
> range, never shares a page with a buffered end.  But where the
> mapping's minimum folio order makes the smallest folio a block larger
> than a page, the page cache holds that block as one folio: an end cut
> on a page boundary inside a block leaves the middle and the buffered
> end in the same folio.  The middle's invalidation then drops the folio
> the end wrote, and whichever of the two neighbouring WRITEs comes second
> reads the block back.
>
> Cut a misaligned end at a multiple of the smallest folio the mapping
> allows as well.  Where that is a page, nothing changes.
>
> Fixes: 80728157d1bd ("NFSD: cut a misaligned direct-mode WRITE at page 
> boundaries")
> Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> ---
>  fs/nfsd/vfs.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index 9219f8eb386d..ade00dabef18 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -1452,13 +1452,15 @@ nfsd_write_dio_iters_init(struct svc_rqst 
> *rqstp, struct svc_fh *fhp,
> 
>  	/*
>  	 * The direct middle invalidates the page cache over its range, so it
> -	 * must not share a page with a buffered end.  An aligned end needs no
> -	 * buffered segment; a misaligned one is cut on a page boundary even
> -	 * where the offset alignment is finer than a page.  A WRITE may end
> -	 * at OFFSET_MAX, so the prefix is sized as a distance rather than
> -	 * by rounding @offset up, which could wrap.
> +	 * must not share a folio with a buffered end.  An aligned end needs no
> +	 * buffered segment; a misaligned one is cut on a boundary of the
> +	 * smallest folio the mapping allows, a page or, where the block is
> +	 * larger, a block, even where the offset alignment is finer.  A WRITE
> +	 * may end at OFFSET_MAX, so the prefix is sized as a distance rather
> +	 * than by rounding @offset up, which could wrap.
>  	 */
> -	cut = max_t(u32, offset_align, PAGE_SIZE);
> +	cut = max3(offset_align, PAGE_SIZE,
> +		   mapping_min_folio_nrbytes(nf->nf_file->f_mapping));
>  	prefix_end = offset;
>  	if (!IS_ALIGNED(offset, offset_align))
>  		prefix_end += min_t(loff_t, cut - (offset & (cut - 1)), total);
> -- 
> 2.52.0

PAGE_SIZE looks dead in the max3(). mapping_min_folio_nrbytes() is always
at least PAGE_SIZE, so the cut is just max(offset_align, min folio). The
new commit message and comment both describe three independent bounds, but
the accurate statement is two: the DIO offset alignment and the mapping's
smallest folio.

Also I would like to keep the helper's unsigned long type instead of
narrowing to u32.

Post an updated version of the v7 patch so we can test and review that
as a single change. This will also archive that revision in the lore
archive. This will likely result in merge conflicts with subsequent
patches in the series. If those are just comment conflicts I can fix
those up when I apply the revised version.

Do not include the new revision of this patch in v9. v9 is now patches
destined for the MM tree(s) while this patch goes to NFSD.


-- 
Chuck Lever (Come to NFS bake-a-thon! https://nfsv4bat.org)