[PATCH v2 1/3] userfaultfd: ensure mremap_userfaultfd_fail() releases mmap_changing

[Mike Rapoport]

From: "Mike Rapoport (Microsoft)" <rppt@xxxxxxxxxx>

Sashiko says:

  mremap_userfaultfd_prep() increments ctx->mmap_changing to stall
  concurrent operations, but mremap_userfaultfd_fail() does not
  decrement it before dropping the context reference.

If an mremap operation fails, ctx->mmap_changing remains elevated. This
will causes subsequent userfaultfd operations like a UFFDIO_COPY to fail
with -EAGAIN.

Decrement ctx->mmap_changing in mremap_userfaultfd_fail().

Link: https://sashiko.dev/#/patchset/20260430113512.115938-1-rppt@xxxxxxxxxx
Fixes: df2cc96e7701 ("userfaultfd: prevent non-cooperative events vs mcopy_atomic races")
Signed-off-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
---
 fs/userfaultfd.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 4b53dc4a3266..ef963a58f1a1 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -786,6 +786,7 @@ void mremap_userfaultfd_fail(struct vm_userfaultfd_ctx *vm_ctx)
 	if (!ctx)
 		return;
 
+	atomic_dec(&ctx->mmap_changing);
 	userfaultfd_ctx_put(ctx);
 }
 
-- 
2.53.0