From: David Hildenbrand <david@redhat.com>
To: Peter Xu <peterx@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, stable@vger.kernel.org
Subject: Re: [PATCH v2 2/2] mm/hugetlb: support write-faults in shared mappings
Date: Thu, 11 Aug 2022 18:24:59 +0200 [thread overview]
Message-ID: <2cb95372-f7dd-61f9-9d2a-461390fb0907@redhat.com> (raw)
In-Reply-To: <YvULP8DtFFhJYNO4@xz-m1.local>
On 11.08.22 15:59, Peter Xu wrote:
> On Thu, Aug 11, 2022 at 12:34:35PM +0200, David Hildenbrand wrote:
>> Reason is that uffd-wp doesn't clear the uffd-wp PTE bit when
>> unregistering and consequently keeps the PTE writeprotected. Reason for
>> this is to avoid the additional overhead when unregistering. Note
>> that this is the case also for !hugetlb and that we will end up with
>> writable PTEs that still have the uffd-wp PTE bit set once we return
>> from hugetlb_wp(). I'm not touching the uffd-wp PTE bit for now, because it
>> seems to be a generic thing -- wp_page_reuse() also doesn't clear it.
>
> This may justify that lazy reset of ptes may not really be a good idea,
> including anonymous. I'm indeed not aware of any app that do frequent
> reg/unreg at least.
Yeah. QEMU snapshots come to mind, but I guess the reg/unreg overhead is
the smallest issue.
>
> I'll prepare a patch to change it from uffd side too.
>
> Thanks again for finding this problem.
YW!
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2022-08-11 16:51 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-11 10:34 [PATCH v2 0/2] mm/hugetlb: fix write-fault handling for shared mappings David Hildenbrand
2022-08-11 10:34 ` [PATCH v2 1/2] mm/hugetlb: fix hugetlb not supporting softdirty tracking David Hildenbrand
2022-08-11 18:27 ` Mike Kravetz
2022-08-11 10:34 ` [PATCH v2 2/2] mm/hugetlb: support write-faults in shared mappings David Hildenbrand
2022-08-11 13:59 ` Peter Xu
2022-08-11 16:24 ` David Hildenbrand [this message]
2022-08-11 18:59 ` Mike Kravetz
2022-08-15 13:35 ` Gerald Schaefer
2022-08-15 15:07 ` David Hildenbrand
2022-08-15 15:59 ` Gerald Schaefer
2022-08-15 18:03 ` David Hildenbrand
2022-08-15 18:38 ` Gerald Schaefer
2022-08-15 21:43 ` Mike Kravetz
2022-08-16 9:33 ` Gerald Schaefer
2022-08-16 20:43 ` David Hildenbrand
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