linux-kernel - Re: [PATCH STABLE 5.10] mm/memory: add non-anonymous page check in the copy_present

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Date:   Thu, 27 Oct 2022 18:32:01 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Peter Xu <peterx@...hat.com>
cc:     Hugh Dickins <hughd@...gle.com>,
        Yuanzheng Song <songyuanzheng@...wei.com>,
        akpm@...ux-foundation.org, gregkh@...uxfoundation.org,
        david@...hat.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH STABLE 5.10] mm/memory: add non-anonymous page check in
 the copy_present_page()

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