[PATCH 03/21] NFSv4/flexfiles: Bound page coalescing on the absolute stripe offset

ff_layout_pg_test() was copied from the files layout, which partitions
data servers relative to pattern_offset and bounds coalescing with a
segment-relative offset.  Flexfiles has no pattern_offset:
nfs4_ff_layout_calc_dss_id() partitions on the absolute file offset
(RFC 8435 Section 6).  So when a layout segment's offset is not
stripe-unit-aligned, the segment-relative coalescing window is shifted
off the absolute stripe grid and a coalesced I/O can straddle a stripe
boundary, sending the bytes past it to the wrong data server.

Bound coalescing on the absolute offset to match calc_dss_id().  Aligned
segments, including all whole-file layouts, are unchanged.

Fixes: 4934ccbeaed3 ("NFSv4/flexfiles: Read path updates for striped layouts")
Cc: [email protected]
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Benjamin Coddington <[email protected]>
---
 fs/nfs/flexfilelayout/flexfilelayout.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/fs/nfs/flexfilelayout/flexfilelayout.c b/fs/nfs/flexfilelayout/flexfilelayout.c
index cce56faa8b70..692ad84047d4 100644
--- a/fs/nfs/flexfilelayout/flexfilelayout.c
+++ b/fs/nfs/flexfilelayout/flexfilelayout.c
@@ -996,7 +996,6 @@ ff_layout_pg_test(struct nfs_pageio_descriptor *pgio, struct nfs_page *prev,
 	unsigned int size;
 	u64 p_stripe, r_stripe;
 	u32 stripe_offset;
-	u64 segment_offset = pgio->pg_lseg->pls_range.offset;
 	u32 stripe_unit = FF_LAYOUT_LSEG(pgio->pg_lseg)->stripe_unit;
 
 	/* calls nfs_generic_pg_test */
@@ -1008,8 +1007,8 @@ ff_layout_pg_test(struct nfs_pageio_descriptor *pgio, struct nfs_page *prev,
 
 	/* see if req and prev are in the same stripe */
 	if (prev) {
-		p_stripe = (u64)req_offset(prev) - segment_offset;
-		r_stripe = (u64)req_offset(req) - segment_offset;
+		p_stripe = (u64)req_offset(prev);
+		r_stripe = (u64)req_offset(req);
 		do_div(p_stripe, stripe_unit);
 		do_div(r_stripe, stripe_unit);
 
@@ -1018,7 +1017,7 @@ ff_layout_pg_test(struct nfs_pageio_descriptor *pgio, struct nfs_page *prev,
 	}
 
 	/* calculate remaining bytes in the current stripe */
-	div_u64_rem((u64)req_offset(req) - segment_offset,
+	div_u64_rem((u64)req_offset(req),
 			stripe_unit,
 			&stripe_offset);
 	WARN_ON_ONCE(stripe_offset > stripe_unit);
-- 
2.53.0