FAILED: patch "[PATCH] NFS/localio: detect a short read or write before the iterator" failed to apply to 6.18-stable tree

PATCH New [email protected] 2 participants 2 messages Nov 4 – Nov 6

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[email protected]

author maintainer

2026-11-04 18:47 UTC #0 lore raw

The patch below does not apply to the 6.18-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x db3ca539c1142d2c5380c5dbd3154ab7bb1c7d77
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026110458-dreamt-steering-e6da@gregkh' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From db3ca539c1142d2c5380c5dbd3154ab7bb1c7d77 Mon Sep 17 00:00:00 2001
From: Mike Snitzer <[email protected]>
Date: Mon, 28 Sep 2026 11:54:26 -0400
Subject: [PATCH] NFS/localio: detect a short read or write before the iterator
has moved

nfs_local_call_read() and nfs_local_call_write() issue a misaligned DIO as up to three segments and must stop at the first short one, or the next segment lands at the wrong file offset. They test for that by comparing the bytes transferred against iov_iter_count() of the segment, read after read_iter() or write_iter() has advanced the iterator, when the count is the residual rather than the request:

    before the call:  count == expected
    after the call:   count == expected - status

The test is therefore status < expected - status, and it fires only when less than half of the segment was transferred. A short I/O that covers between 50% and 99% of a segment slips through: the loop moves on with ki_pos advanced by the short amount, the next segment is issued at the wrong offset, and the header's byte count is over-reported to the caller. A write that slips through also never sets NFS_CONTEXT_WRITE_SYNC, which is what makes the writes that follow a short one synchronous.

Take the segment's count before issuing it and compare against that.

This is the same defect that commit 250ec14932d5 ("nfsd: fix partial-write detection in nfsd_direct_write") fixed in NFSD's version of this loop.

Fixes c817248fc831 ("nfs/localio: add proper O_DIRECT support for READ and WRITE")

Fixes d0497dd27452 ("nfs/localio: backfill missing partial read support for misaligned DIO")

Cc [email protected]

Assisted-by Claude:claude-fable-5-1

Signed-off-by Mike Snitzer <[email protected]>

Signed-off-by Anna Schumaker <[email protected]>

fs/nfs/localio.c +10 -5

index 63c38dea50cc..9fcae2391b72 100644
@@ -674,6 +674,8 @@ static void nfs_local_call_read(struct work_struct *work)
 
 	n_iters = atomic_read(&iocb->n_iters);
 	for (int i = 0; i < n_iters ; i++) {
+		size_t expected;
+
 		if (iocb->iter_is_dio_aligned[i]) {
 			iocb->kiocb.ki_flags |= IOCB_DIRECT;
 			/* Only use AIO completion if DIO-aligned segment is last */

@@ -684,6 +686,8 @@ static void nfs_local_call_read(struct work_struct *work)
 		} else
 			iocb->kiocb.ki_flags &= ~IOCB_DIRECT;
 
+		/* read_iter() advances the iterator: measure it beforehand */
+		expected = iov_iter_count(&iocb->iters[i]);
 		scoped_with_creds(filp->f_cred)
 			status = filp->f_op->read_iter(&iocb->kiocb, &iocb->iters[i]);

@@ -691,7 +695,7 @@ static void nfs_local_call_read(struct work_struct *work)
 			continue;
 		/* Break on completion, errors, or short reads */
 		if (nfs_local_pgio_done(iocb, status) || status < 0 ||
-		    (size_t)status < iov_iter_count(&iocb->iters[i])) {
+		    (size_t)status < expected) {
 			nfs_local_read_iocb_done(iocb);
 			break;
 		}

@@ -890,7 +894,7 @@ static void nfs_local_call_write(struct work_struct *work)
 	file_start_write(filp);
 	n_iters = atomic_read(&iocb->n_iters);
 	for (int i = 0; i < n_iters ; i++) {
-		size_t icount;
+		size_t expected;
 
 		if (iocb->iter_is_dio_aligned[i]) {
 			iocb->kiocb.ki_flags |= IOCB_DIRECT;

@@ -902,16 +906,17 @@ static void nfs_local_call_write(struct work_struct *work)
 		} else
 			iocb->kiocb.ki_flags &= ~IOCB_DIRECT;
 
+		/* write_iter() advances the iterator: measure it beforehand */
+		expected = iov_iter_count(&iocb->iters[i]);
 		scoped_with_creds(filp->f_cred)
 			status = filp->f_op->write_iter(&iocb->kiocb, &iocb->iters[i]);
 
 		if (status == -EIOCBQUEUED)
 			continue;
 		/* Break on completion, errors, or short writes */
-		icount = iov_iter_count(&iocb->iters[i]);
 		if (nfs_local_pgio_done(iocb, status) || status < 0 ||
-		    (size_t)status < icount) {
-			if ((size_t)status < icount) {
+		    (size_t)status < expected) {
+			if ((size_t)status < expected) {
 				struct nfs_lock_context *ctx =
 					iocb->hdr->req->wb_lock_context;

Sasha Levin

maintainer

2026-11-06 01:53 UTC #1 lore raw

PATCH 1/1 NFS/localio: detect a short read or write before the iterator has moved

From: Mike Snitzer <[email protected]>

[ Upstream commit db3ca539c1142d2c5380c5dbd3154ab7bb1c7d77 ]

    before the call:  count == expected
    after the call:   count == expected - status

Take the segment's count before issuing it and compare against that.

This is the same defect that commit 250ec14932d5 ("nfsd: fix partial-write detection in nfsd_direct_write") fixed in NFSD's version of this loop.

Fixes c817248fc831 ("nfs/localio: add proper O_DIRECT support for READ and WRITE")

Fixes d0497dd27452 ("nfs/localio: backfill missing partial read support for misaligned DIO")

Cc [email protected]

Assisted-by Claude:claude-fable-5-1

Signed-off-by Mike Snitzer <[email protected]>

Signed-off-by Anna Schumaker <[email protected]>

[ sashal: adapted completion handling to the three-argument nfs_local_pgio_done() helper using force_done ]

Signed-off-by Sasha Levin <[email protected]>

---
 fs/nfs/localio.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

fs/nfs/localio.c +10 -2

index 3c5891d3ab897..3138439d19109 100644
@@ -682,6 +682,8 @@ static void nfs_local_call_read(struct work_struct *work)
 
 	n_iters = atomic_read(&iocb->n_iters);
 	for (int i = 0; i < n_iters ; i++) {
+		size_t expected;
+
 		if (iocb->iter_is_dio_aligned[i]) {
 			iocb->kiocb.ki_flags |= IOCB_DIRECT;
 			/* Only use AIO completion if DIO-aligned segment is last */

@@ -692,12 +694,14 @@ static void nfs_local_call_read(struct work_struct *work)
 		} else
 			iocb->kiocb.ki_flags &= ~IOCB_DIRECT;
 
+		/* read_iter() advances the iterator: measure it beforehand */
+		expected = iov_iter_count(&iocb->iters[i]);
 		save_cred = override_creds(filp->f_cred);
 		status = filp->f_op->read_iter(&iocb->kiocb, &iocb->iters[i]);
 		revert_creds(save_cred);
 
 		if (status != -EIOCBQUEUED) {
-			if (unlikely(status >= 0 && status < iocb->iters[i].count))
+			if (unlikely(status >= 0 && (size_t)status < expected))
 				force_done = true; /* Partial read */
 			if (nfs_local_pgio_done(iocb, status, force_done)) {
 				nfs_local_read_iocb_done(iocb);

@@ -878,6 +882,8 @@ static void nfs_local_call_write(struct work_struct *work)
 	file_start_write(filp);
 	n_iters = atomic_read(&iocb->n_iters);
 	for (int i = 0; i < n_iters ; i++) {
+		size_t expected;
+
 		if (iocb->iter_is_dio_aligned[i]) {
 			iocb->kiocb.ki_flags |= IOCB_DIRECT;
 			/* Only use AIO completion if DIO-aligned segment is last */

@@ -888,12 +894,14 @@ static void nfs_local_call_write(struct work_struct *work)
 		} else
 			iocb->kiocb.ki_flags &= ~IOCB_DIRECT;
 
+		/* write_iter() advances the iterator: measure it beforehand */
+		expected = iov_iter_count(&iocb->iters[i]);
 		save_cred = override_creds(filp->f_cred);
 		status = filp->f_op->write_iter(&iocb->kiocb, &iocb->iters[i]);
 		revert_creds(save_cred);
 
 		if (status != -EIOCBQUEUED) {
-			if (unlikely(status >= 0 && status < iocb->iters[i].count))
+			if (unlikely(status >= 0 && (size_t)status < expected))
 				force_done = true; /* Partial write */
 			if (nfs_local_pgio_done(iocb, status, force_done)) {
 				nfs_local_write_iocb_done(iocb);

Signature · 1 line

2.53.0