linux-kernel - Re: [PATCH v1] selftests: mm: gup_longterm: test unsharing logic when R/O pinning

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Date: Tue, 30 Apr 2024 09:36:05 -0400
From: Peter Xu <peterx@...hat.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-kselftest@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH v1] selftests: mm: gup_longterm: test unsharing logic
 when R/O pinning

On Tue, Apr 30, 2024 at 03:15:08PM +0200, David Hildenbrand wrote:
> In our FOLL_LONGTERM tests, we prefault the page tables for the GUP-fast
> test cases to be able to find a PTE and exercise the "longterm pinning
> allowed" logic on the GUP-fast path where possible.
> 
> For now, we always prefault the page tables writable, resulting in PTEs
> that are writable.
> 
> Let's cover more cases to also test if our unsharing logic works as
> expected (and is able to make progress when there is nothing to unshare)
> by mprotect'ing the range R/O when R/O-pinning, so we don't get PTEs
> that are writable.
> 
> This change would have found an issue introduced by commit a12083d721d7
> ("mm/gup: handle hugepd for follow_page()"), whereby R/O pinning was not
> able to make progress in all cases, because unsharing logic was not
> provided with the VMA to decide at some point that long-term R/O pinning
> a !anon page is fine.
> 
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Shuah Khan <shuah@...nel.org>
> Cc: Peter Xu <peterx@...hat.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>

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

Thanks,

-- 
Peter Xu