Re: [patch for-4.7] mm, compaction: prevent VM_BUG_ON when terminating freeing scanner

[Greg KH]

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@xxxxxxxxxx>
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
> ---
>  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>