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

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Date:   Thu, 2 Mar 2023 17:29:50 -0500
From:   Peter Xu <peterx@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Jerome Glisse <jglisse@...hat.com>, Shaohua Li <shli@...com>
Subject: Re: [PATCH v1] mm/userfaultfd: propagate uffd-wp bit when
 PTE-mapping the huge zeropage

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.

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

Thanks,

-- 
Peter Xu