linux-kernel - Re: [PATCH v1] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA

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Message-ID: <6209d614-b1f2-4501-6b8a-6d4095c309eb@redhat.com>
Date:   Fri, 9 Dec 2022 09:07:44 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Ives van Hoorne <ives@...esandbox.io>,
        stable@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hugh@...itas.com>,
        Alistair Popple <apopple@...dia.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH v1] mm/userfaultfd: enable writenotify while
 userfaultfd-wp is enabled for a VMA

On 08.12.22 21:21, Peter Xu wrote:
> On Thu, Dec 08, 2022 at 03:06:06PM -0500, Peter Xu wrote:
>> On Thu, Dec 08, 2022 at 05:44:35PM +0100, David Hildenbrand wrote:
>>> I'll wait for some more (+retest) before I resend tomorrow.
>>
>> One more thing just to double check:
>>
>> It's 6a56ccbcf6c6 ("mm/autonuma: use can_change_(pte|pmd)_writable() to
>> replace savedwrite", 2022-11-30) that just started to break uffd-wp on
>> numa, am I right?
>>
>> With the old code, pte_modify() will persist uffd-wp bit, afaict, and we
>> used to do savedwrite for numa hints.  That all look correct to me until
>> the savedwrite removal patchset with/without vm_page_prot changes.
>>
>> If that's the case, we'd better also mention that in the commit message and
>> has another Fixes: for that one to be clear.
> 
> Nah, never mind.  I think the savedwrite will not guarantee pte write
> protected just like the migration path.  The commit message is correct.

Right, the problem is not the uffd-wp bit getting lost, but the write 
bit getting set, which is independent of 6a56ccbcf6c6. Thanks for 
double-checking 6a56ccbcf6c6.

-- 
Thanks,

David / dhildenb