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Date:   Thu, 27 Oct 2022 18:32:01 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Peter Xu <peterx@redhat.com>
cc:     Hugh Dickins <hughd@google.com>,
        Yuanzheng Song <songyuanzheng@huawei.com>,
        akpm@linux-foundation.org, gregkh@linuxfoundation.org,
        david@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org
Subject: Re: [PATCH STABLE 5.10] mm/memory: add non-anonymous page check in
 the copy_present_page()
In-Reply-To: <Y1sMk30wS+1uH/hc@x1n>
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References: <20221024094911.3054769-1-songyuanzheng@huawei.com> <3823471f-6dda-256e-e082-718879c05449@google.com> <Y1nRiJ1LYB62uInn@x1n> <fffefe4-adce-a7d-23e0-9f8afc7ce6cf@google.com> <Y1qdY8oUlUvWl067@x1n> <8aad435-bdc6-816f-2fe4-efe53abd6e5@google.com>
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Reinstating Cc stable, which I removed just before the discussion settled.

On Thu, 27 Oct 2022, Peter Xu wrote:
> ...
> 
> After a re-read and 2nd thought, I think David has a valid point in that we
> shouldn't have special handling of !anon pages on CoW during fork(),
> because that seems to be against the fundamental concept of fork().
> 
> So now I think I agree the !Anon original check does look a bit cleaner,
> and also make fork() behavior matching with the old/new kernels, irrelevant
> of the pin mess.

Thanks Peter.  So Yuanzheng's patch for 5.10 is exactly right.

Sorry for leading everyone astray: my mistake was to suppose that
its !PageAnon check was simply to avoid the later BUG_ON(!anon_vma):
whereas David and Peter now agree that it actually corrects the
semantics for fork() on file pages.

I lift my hold on Yuanzheng's patch: nobody actually said "Acked-by",
but I think the discussion and resolution have given better than that.
(No 3rd thoughts please!)

Hugh