linux-kernel - Re: [PATCH v2 2/2] mm/hugetlb: support write-faults in shared mappings

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Date:   Thu, 11 Aug 2022 09:59:27 -0400
From:   Peter Xu <peterx@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        stable@...r.kernel.org
Subject: Re: [PATCH v2 2/2] mm/hugetlb: support write-faults in shared
 mappings

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.

I'll prepare a patch to change it from uffd side too.

Thanks again for finding this problem.

-- 
Peter Xu