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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 5/11/26 12:48, Mike Rapoport wrote:
> On Mon, May 11, 2026 at 11:15:34AM +0200, David Hildenbrand (Arm) wrote:
>> On 5/1/26 16:54, Mike Rapoport wrote:
>>> From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>
>>>
>>> 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?
> 
> Yes.
>  
>>> 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")
>>> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
>>> ---
>>>  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?
> 
> Yeah, we could.
In any case

Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David