From: "Mike Rapoport (Microsoft)" 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@kernel.org Fixes: df2cc96e7701 ("userfaultfd: prevent non-cooperative events vs mcopy_atomic races") Reviewed-by: David Hildenbrand (Arm) Signed-off-by: Mike Rapoport (Microsoft) --- I split the fix from the code movement series, will be easier to everyone :) v2 changes: * VM_WARN() if mmap_changing is going negative v1: https://lore.kernel.org/all/20260501145433.156211-1-rppt@kernel.org (patch 1/3) fs/userfaultfd.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 4b53dc4a3266..390e4b7d9cb9 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -786,6 +786,8 @@ void mremap_userfaultfd_fail(struct vm_userfaultfd_ctx *vm_ctx) if (!ctx) return; + atomic_dec(&ctx->mmap_changing); + VM_WARN_ON_ONCE(atomic_read(&ctx->mmap_changing) < 0); userfaultfd_ctx_put(ctx); } base-commit: 972c53e0ec3abfc6f5fe2cb503640710fb23cf95 -- 2.53.0