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References: <20230104225207.1066932-1-peterx@redhat.com> <20230104225207.1066932-4-peterx@redhat.com>
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From:   James Houghton <jthoughton@google.com>
Date:   Thu, 5 Jan 2023 01:52:56 +0000
Message-ID: <CADrL8HVGNPWrhBNgpofgt1kpc5-rCm3GKAFwbupm6_3i6=OBaA@mail.gmail.com>
Subject: Re: [PATCH 3/3] mm/uffd: Detect pgtable allocation failures
To:     Peter Xu <peterx@redhat.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Andrew Morton <akpm@linux-foundation.org>
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On Wed, Jan 4, 2023 at 10:52 PM Peter Xu <peterx@redhat.com> wrote:
>
> Before this patch, when there's any pgtable allocation issues happened
> during change_protection(), the error will be ignored from the syscall.
> For shmem, there will be an error dumped into the host dmesg.  Two issues
> with that:
>
>   (1) Doing a trace dump when allocation fails is not anything close to
>       grace..
>
>   (2) The user should be notified with any kind of such error, so the user
>       can trap it and decide what to do next, either by retrying, or stop
>       the process properly, or anything else.
>
> For userfault users, this will change the API of UFFDIO_WRITEPROTECT when
> pgtable allocation failure happened.  It should not normally break anyone,
> though.  If it breaks, then in good ways.
>
> One man-page update will be on the way to introduce the new -ENOMEM for
> UFFDIO_WRITEPROTECT.  Not marking stable so we keep the old behavior on the
> 5.19-till-now kernels.
>
> Reported-by: James Houghton <jthoughton@google.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>

Acked-by: James Houghton <jthoughton@google.com>

Thanks Peter! :)