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

[Mike Snitzer <snitzer@kernel.org>]

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