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

[David Hildenbrand (Arm)]

On 5/1/26 16:54, Mike Rapoport wrote:
> 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.
> 

Sounds like we should CC stable?

> 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);

I'll note that other users have a

	VM_WARN_ON_ONCE(atomic_read(&ctx->mmap_changing) < 0);

In there. Likely we should do the same?

-- 
Cheers,

David