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

[Mike Snitzer <snitzer@kernel.org>]

On Fri, Oct 23, 2026 at 01:45:21PM -0400, Chuck Lever wrote:
> 
> 
> 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.

v7 didn't have this 1/7 patch.  But seems you're saying the fix for
nfsd-testing commit 80728157d1bd (which started as v4 1/10) would be
to have max_t() use mapping_min_folio_nrbytes(nf->nf_file->f_mapping)
instead of PAGE_SIZE?

I think you're saying post a revised version of commit 80728157d1bd as
a standalone NFSD patch?  I can check if it causes the other patches
you've staged to have conflicts (but like you say, should be simple to
resolve).

Thanks,
Mike