Skip to contentSkip to conversation

userfaultfd: reset err to be 0 when move_pages_ptes succeeded

PATCH Applied Foxie Flakey 4 participants 5 messages Aug 19

Subsystems (2)
Memory management userfaultfd
lore
Share & tools
lore

Foxie Flakey

author
#0 lore raw

PATCH v3 1/1 userfaultfd: reset err to be 0 when move_pages_ptes succeeded

During move_pages() operation, when move_pages_ptes() returns EAGAIN, the error code is not cleared even after we processed it. This leads to a successful retry but then the same pages are retried again due to the stale error code. This time move fails because pages are already moved, loop is terminated and move_pages() reports a failure. Clear the error code once we processes EAGAIN.

Fixes 50944692052b ("userfaultfd: opportunistic TLB-flush batching for present pages in MOVE")
Assisted-by ChatGPT:GPT-5.6-Luna
Signed-off-by Bryan Lim <[email protected]>

---
Changes in v3:
- Updated to use proper sign off.

Changes in v2:
- Updated on description to be easier to understood
- Reset err before retrying, instead of fixing right at the move_pages_ptes branch.
- Added Assisted-by, received an answer few days ago from linux-newbies IRC that even I just used AI/LLM as "fast path" of git bisect. I still need to add it following
https://docs.kernel.org/process/coding-assistants.html doc.

---
 mm/userfaultfd.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
mm/userfaultfd.c +3 -1
index c3adedaaf7d5..ec5d511bd179 100644
@@ -2085,8 +2085,10 @@ static ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
 		}
 
 		if (err) {
-			if (err == -EAGAIN)
+			if (err == -EAGAIN) {
+				err = 0;
 				continue;
+			}
 			break;
 		}
Signature · 1 line
2.55.0
maintainer
#1 lore raw
Quoted 11 lines · On Wed, 19 Aug 2026 10:08:24 +0700 (WIB) Foxie Flakey <[email protected]>…

During move_pages() operation, when move_pages_ptes() returns EAGAIN, the error code is not cleared even after we processed it. This leads to a successful retry but then the same pages are retried again due to the stale error code. This time move fails because pages are already moved, loop is terminated and move_pages() reports a failure. Clear the error code once we processes EAGAIN.

Fixes 50944692052b ("userfaultfd: opportunistic TLB-flush batching for present pages in MOVE")
Assisted-by ChatGPT:GPT-5.6-Luna
Signed-off-by Bryan Lim <[email protected]>

Thanks, Bryan.

More paperwork: the signoff is supposed to match the Author:. The way to do this is to add an explicit From: line at start-of-changelog. If this is not present, we fall back to the email envelope's From:.

mm/userfaultfd.c static ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start, · line 2093

Quoted 13 lines
mm/userfaultfd.c +3 -1
@@ -2085,8 +2085,10 @@ static ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
 		}
 
 		if (err) {
-			if (err == -EAGAIN)
+			if (err == -EAGAIN) {
+				err = 0;
 				continue;
+			}
 			break;
 		}

I'll queue it, with an adjusted From:, as above.

Could reviewers please advise on whether we should backport this into -stable kernels?

author
#2 lore raw
Quoted 10 lines · Andrew Morton
Quoted 8 lines · On Wed, 19 Aug 2026 10:08:24 +0700 (WIB) Foxie Flakey <[email protected]>…

During move_pages() operation, when move_pages_ptes() returns EAGAIN, the error code is not cleared even after we processed it. This leads to a successful retry but then the same pages are retried again due to the stale error code. This time move fails because pages are already moved, loop is terminated and move_pages() reports a failure. Clear the error code once we processes EAGAIN.

Quoted 5 lines
Quoted 3 lines
Fixes 50944692052b ("userfaultfd: opportunistic TLB-flush batching for present pages in MOVE")
Assisted-by ChatGPT:GPT-5.6-Luna
Signed-off-by Bryan Lim <[email protected]>

Thanks, Bryan.

No, problem. Thank you for your time. If you don't mind this, I would prefer keep to be called as Foxie unless its required to be Bryan Lim like the sign off. Sorry, if it took a while for me to understand.

Quoted 3 lines

More paperwork: the signoff is supposed to match the Author:. The way to do this is to add an explicit From: line at start-of-changelog. If this is not present, we fall back to the email envelope's From:.

Noted. I'll keep that in mind for my future patches.

Quoted 5 lines
Quoted 5 lines
mm/userfaultfd.c +0 -0
@@ -2085,8 +2085,10 @@ static ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
 		}
Quoted 10 lines

mm/userfaultfd.c static ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start, · line 2093

Quoted 8 lines
mm/userfaultfd.c +3 -1
 		if (err) {
-			if (err == -EAGAIN)
+			if (err == -EAGAIN) {
+				err = 0;
 				continue;
+			}
 			break;
 		}

I'll queue it, with an adjusted From:, as above.

Thank you!

Quoted 4 lines

Could reviewers please advise on whether we should backport this into -stable kernels?

Reviewed-by
#3 lore raw
In reply to Andrew Morton
Quoted 11 lines · Andrew Morton
Quoted 8 lines · On Wed, 19 Aug 2026 10:08:24 +0700 (WIB) Foxie Flakey <[email protected]>…

During move_pages() operation, when move_pages_ptes() returns EAGAIN, the error code is not cleared even after we processed it. This leads to a successful retry but then the same pages are retried again due to the stale error code. This time move fails because pages are already moved, loop is terminated and move_pages() reports a failure. Clear the error code once we processes EAGAIN.

Quoted 3 lines
Quoted 3 lines
Fixes 50944692052b ("userfaultfd: opportunistic TLB-flush batching for present pages in MOVE")
Assisted-by ChatGPT:GPT-5.6-Luna
Signed-off-by Bryan Lim <[email protected]>
Reviewed-by Suren Baghdasaryan <[email protected]>
Quoted 23 lines

Thanks, Bryan.

More paperwork: the signoff is supposed to match the Author:. The way to do this is to add an explicit From: line at start-of-changelog. If this is not present, we fall back to the email envelope's From:.

mm/userfaultfd.c static ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start, · line 2093

Quoted 13 lines
mm/userfaultfd.c +3 -1
@@ -2085,8 +2085,10 @@ static ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
              }
 
              if (err) {
-                     if (err == -EAGAIN)
+                     if (err == -EAGAIN) {
+                             err = 0;
                              continue;
+                     }
                      break;
              }

I'll queue it, with an adjusted From:, as above.

Quoted 2 lines

Could reviewers please advise on whether we should backport this into -stable kernels?

Yes please. The first version where the bug appears is 6.18. Thanks!

Quoted 2 lines
maintainer Acked-by on v3 1/1
#4 lore raw
In reply to Foxie Flakey

Hi Foxie,

Quoted 11 lines · Foxie Flakey

During move_pages() operation, when move_pages_ptes() returns EAGAIN, the error code is not cleared even after we processed it. This leads to a successful retry but then the same pages are retried again due to the stale error code. This time move fails because pages are already moved, loop is terminated and move_pages() reports a failure. Clear the error code once we processes EAGAIN.

Fixes 50944692052b ("userfaultfd: opportunistic TLB-flush batching for present pages in MOVE")
Assisted-by ChatGPT:GPT-5.6-Luna
Signed-off-by Bryan Lim <[email protected]>

With the authorship changes that Andrew requested

Acked-by Mike Rapoport (Microsoft) <[email protected]>
Quoted 36 lines

---
Changes in v3:
- Updated to use proper sign off.

Changes in v2:
- Updated on description to be easier to understood
- Reset err before retrying, instead of fixing right at the move_pages_ptes branch.
- Added Assisted-by, received an answer few days ago from linux-newbies IRC that even I just used AI/LLM as "fast path" of git bisect. I still need to add it following
https://docs.kernel.org/process/coding-assistants.html doc.

---
 mm/userfaultfd.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
mm/userfaultfd.c +3 -1
index c3adedaaf7d5..ec5d511bd179 100644
@@ -2085,8 +2085,10 @@ static ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
 		}
 
 		if (err) {
-			if (err == -EAGAIN)
+			if (err == -EAGAIN) {
+				err = 0;
 				continue;
+			}
 			break;
 		}
Signature · 1 line
2.55.0
Signature · 2 lines
Sincerely yours,
Mike.