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

[Peter Xu]

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