Re: + mm-compaction-avoid-premature-range-skip-in-isolate_migratepages_range.patch added to -mm tree

[Vlastimil Babka]

On 10/24/2014 05:00 AM, Joonsoo Kim wrote:
> On Thu, Oct 23, 2014 at 10:39:45AM +0200, Vlastimil Babka wrote:
> > On 10/23/2014 10:15 AM, Joonsoo Kim wrote:
> > > On Tue, Oct 14, 2014 at 01:53:44PM -0700, akpm@xxxxxxxxxxxxxxxxxxxx wrote:
> > > > The patch titled
> > > >       Subject: mm/compaction.c: avoid premature range skip in isolate_migratepages_range
> > > > has been added to the -mm tree.  Its filename is
> > > >       mm-compaction-avoid-premature-range-skip-in-isolate_migratepages_range.patch
> > > >
> > > > This patch should soon appear at
> > > >      http://ozlabs.org/~akpm/mmots/broken-out/mm-compaction-avoid-premature-range-skip-in-isolate_migratepages_range.patch
> > > > and later at
> > > >      http://ozlabs.org/~akpm/mmotm/broken-out/mm-compaction-avoid-premature-range-skip-in-isolate_migratepages_range.patch
> > > >
> > > > Before you just go and hit "reply", please:
> > > >     a) Consider who else should be cc'ed
> > > >     b) Prefer to cc a suitable mailing list as well
> > > >     c) Ideally: find the original patch on the mailing list and do a
> > > >        reply-to-all to that, adding suitable additional cc's
> > > >
> > > > *** Remember to use Documentation/SubmitChecklist when testing your code ***
> > > >
> > > > The -mm tree is included into linux-next and is updated
> > > > there every 3-4 working days
> > > >
> > > > ------------------------------------------------------
> > > > From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > > > Subject: mm/compaction.c: avoid premature range skip in isolate_migratepages_range
> > > >
> > > > commit edc2ca612496 ("mm, compaction: move pageblock checks up from
> > > > isolate_migratepages_range()") commonizes isolate_migratepages variants
> > > > and make them use isolate_migratepages_block().
> > > >
> > > > isolate_migratepages_block() could stop the execution when enough pages
> > > > are isolated, but, there is no code in isolate_migratepages_range() to
> > > > handle this case.  In the result, even if isolate_migratepages_block()
> > > > returns prematurely without checking all pages in the range,
> > > >
> > > > isolate_migratepages_block() is called repeately on the following
> > > > pageblock and some pages in the previous range are skipped to check.
> > > > Then, CMA is failed frequently due to this fact.
> > > >
> > > > To fix this problem, this patch let isolate_migratepages_range() know the
> > > > situation that enough pages are isolated and stop the isolation in that
> > > > case.
> > > >
> > > > Note that isolate_migratepages() has no such problem, because, it always
> > > > stops the isolation after just one call of isolate_migratepages_block().
> > > >
> > > > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > > > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > > > Cc: David Rientjes <rientjes@xxxxxxxxxx>
> > > > Cc: Minchan Kim <minchan@xxxxxxxxxx>
> > > > Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
> > > > Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> > > > Cc: Christoph Lameter <cl@xxxxxxxxx>
> > > > Cc: Rik van Riel <riel@xxxxxxxxxx>
> > > > Cc: Mel Gorman <mgorman@xxxxxxx>
> > > > Cc: Zhang Yanfei <zhangyanfei@xxxxxxxxxxxxxx>
> > > > Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> >
> > Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> >
> > Sorry for the trouble. But I think a more robust and future-proof fix
> > would be a check such as: if (pfn < block_end_pfn) break;
> > This should catch any reason where isolate_migratepages_block() did not
> > finish whole pageblock, and which was not fatal enough to return pfn==0.
> > However currently this seems to happen only due to isolating too much,
> > so your patch should work.
> > So it's up to you if you want to make the check more generic now, or
> > later after this bug is fixed for 3.18.
>
> 'if (pfn < block_end_pfn) break;' has one problem.
> If we have enough isolated pages and reach at block_end_pfn,
> we can't stop with above check.

Oh, right.

> More proper check may be as following.
> 'if (pfn < block_end_pfn ||
> cc->nr_migratepages == COMPACT_CLUSTER_MAX) break;'
> But, as you mentioned, there is no case where 'pfn < block_end_pfn'
> now, so I'd like to remain the patch as is.

Yep, that would only make things uglier :/ So I'm for merging your patch 
in 3.18 cycle to prevent breakage.

> Thanks.

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