Re: [PATCH v1] mm/userfaultfd: propagate uffd-wp bit when PTE-mapping the huge zeropage

From: David Hildenbrand <david@redhat.com>
To: Peter Xu <peterx@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Jerome Glisse <jglisse@redhat.com>, Shaohua Li <shli@fb.com>
Subject: Re: [PATCH v1] mm/userfaultfd: propagate uffd-wp bit when PTE-mapping the huge zeropage
Date: Fri, 3 Mar 2023 10:12:26 +0100	[thread overview]
Message-ID: <95b0a3dd-b6f9-0305-759c-b715359b0cab@redhat.com> (raw)
In-Reply-To: <ZAEjXqNH+U8p9fOG@x1n>

On 02.03.23 23:29, Peter Xu wrote:
> On Thu, Mar 02, 2023 at 06:54:23PM +0100, David Hildenbrand wrote:
>> Currently, we'd lose the userfaultfd-wp marker when PTE-mapping a huge
>> zeropage, resulting in the next write faults in the PMD range
>> not triggering uffd-wp events.
>>
>> Various actions (partial MADV_DONTNEED, partial mremap, partial munmap,
>> partial mprotect) could trigger this. However, most importantly,
>> un-protecting a single sub-page from the userfaultfd-wp handler when
>> processing a uffd-wp event will PTE-map the shared huge zeropage and
>> lose the uffd-wp bit for the remainder of the PMD.
>>
>> Let's properly propagate the uffd-wp bit to the PMDs.
> 
> Ouch.. I thought this was reported once, probably it fell through the
> cracks.

Yes, I reported it a while ago, but our understanding back then was that 
primarily MADV_DONTNEED would trigger it (which my reproducer back then 
did), and e.g., QEMU would make sure to not have concurrent 
MADV_DONTNEED while doing background snapshots.

I realized only yesterday when retesting my patch that that a simple 
unprotect is already sufficient to mess it up.

> 
> Acked-by: Peter Xu <peterx@redhat.com>

Thanks!

-- 
Thanks,

David / dhildenb