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SubjectRe: [patch for-4.7] mm, compaction: prevent VM_BUG_ON when terminating freeing scanner
On Wed, Jun 29, 2016 at 02:47:20PM -0700, David Rientjes wrote:
> It's possible to isolate some freepages in a pageblock and then fail
> split_free_page() due to the low watermark check. In this case, we hit
> VM_BUG_ON() because the freeing scanner terminated early without a
> contended lock or enough freepages.
>
> This should never have been a VM_BUG_ON() since it's not a fatal
> condition. It should have been a VM_WARN_ON() at best, or even handled
> gracefully.
>
> Regardless, we need to terminate anytime the full pageblock scan was not
> done. The logic belongs in isolate_freepages_block(), so handle its state
> gracefully by terminating the pageblock loop and making a note to restart
> at the same pageblock next time since it was not possible to complete the
> scan this time.
>
> Reported-by: Minchan Kim <minchan@kernel.org>
> Signed-off-by: David Rientjes <rientjes@google.com>
> ---
> Note: I really dislike the low watermark check in split_free_page() and
> consider it poor software engineering. The function should split a free
> page, nothing more. Terminating memory compaction because of a low
> watermark check when we're simply trying to migrate memory seems like an
> arbitrary heuristic. There was an objection to removing it in the first
> proposed patch, but I think we should really consider removing that
> check so this is simpler.
>
> mm/compaction.c | 37 +++++++++++++++----------------------
> 1 file changed, 15 insertions(+), 22 deletions(-)

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree. Please read Documentation/stable_kernel_rules.txt
for how to do this properly.

</formletter>

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