[PATCH 04/21] NFSv4/filelayout: Anchor page coalescing on pattern_offset

filelayout_pg_test() bounds page coalescing to a single stripe unit
using an offset relative to pls_range.offset, but the data server is
selected by nfs4_fl_calc_j_index() using an offset relative to
pattern_offset.  When a segment's pattern_offset and range offset are
not congruent modulo the stripe unit, the coalescing window is shifted
off the DS-selection grid, so a coalesced I/O can straddle a stripe
boundary and send the bytes past it to the wrong data server.

Anchor coalescing on pattern_offset to match nfs4_fl_calc_j_index().
Stripe-congruent segments, including the common whole-file pattern_offset
0 case, are unchanged.

Fixes: c6194271f94b ("pnfs: filelayout: support non page aligned layouts")
Cc: [email protected]
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Benjamin Coddington <[email protected]>
---
 fs/nfs/filelayout/filelayout.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/nfs/filelayout/filelayout.c b/fs/nfs/filelayout/filelayout.c
index 72e20b56fbc7..0d53277f972e 100644
--- a/fs/nfs/filelayout/filelayout.c
+++ b/fs/nfs/filelayout/filelayout.c
@@ -796,7 +796,7 @@ filelayout_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;
+	u64 pattern_offset = FILELAYOUT_LSEG(pgio->pg_lseg)->pattern_offset;
 	u32 stripe_unit = FILELAYOUT_LSEG(pgio->pg_lseg)->stripe_unit;
 
 	/* calls nfs_generic_pg_test */
@@ -808,8 +808,8 @@ filelayout_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) - pattern_offset;
+		r_stripe = (u64)req_offset(req) - pattern_offset;
 		do_div(p_stripe, stripe_unit);
 		do_div(r_stripe, stripe_unit);
 
@@ -818,7 +818,7 @@ filelayout_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) - pattern_offset,
 			stripe_unit,
 			&stripe_offset);
 	WARN_ON_ONCE(stripe_offset > stripe_unit);
-- 
2.53.0