Re: [PATCH 3/3] mm/uffd: Detect pgtable allocation failures

[James Houghton <jthoughton@google.com>]

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! :)