Re: [PATCH RFC] mm: compaction: avoid migrating non-cma pages to a cma area

Vlastimil Babka
2020-04-17T08:37:20Z

On 4/14/20 5:42 PM, Roman Gushchin wrote:
> On Tue, Apr 14, 2020 at 01:49:45PM +0200, Vlastimil Babka wrote:
>
> Hello, Vlastimil!
>
> Thank you for looking into it.
>
>> Hm I think I'd rather make such pages really unmovable (by a pin?) than deny the
>> whole CMA area to compaction. Would it be feasible?
>
> Well, it's an option too, however I'm not sure it's the best one.
> First, because these pages can be moved quite often, making
> them completely unmovable will make the compaction less efficient.
> Second, because it's not only about these pages, but in general
> about migrating pages into a cma area without a clear need.
>
> As I wrote in the commit log, if a cma area is located close to end
> of a node (which seems to be default at least on x86 without a movable
> zone), compaction can fill it quite aggressively. And it might bring
> some hot pages (e.g. executable pagecache pages), which will cause
> cma allocation failures. I've seen something like this in our prod.

Hmm, I see.

>>
>> > Compaction moves them to the hugetlb_cma area, and then sometimes
>> > the cma allocator fails to move them back from the cma area. It
>> > results in failures of gigantic hugepages allocations.
>> >
>> > Also in general cma areas are reserved close to the end of a zone,
>> > and it's where compaction tries to migrate pages. It means
>> > compaction will aggressively fill cma areas, which makes not much
>> > sense.
>>
>> So now the free page scanner will have to skip those areas, which is not much
>> effective. But I suspect a worse problem in __compaction_suitable() which will
>> now falsely report that there are enough free pages, so compaction will start
>> but fail to do anytning. Minimally the __zone_watermark_ok() check there would
>> have to lose ALLOC_CMA, but there might be other similar checks that would need
>> adjusting.
>
> A really good point! I've looked around for any other checks, but haven't found
> anything. Please, find an updated version of the patch below.

Technically there's also __isolate_free_page() using ALLOC_CMA for watermark
check, but it's shared by compaction and alloc_contig_range(), so we can't just
remove ALLOC_CMA from there. It's probably not worth to complicate it though. If
we pass compaction_suitable() without ALLOC_CMA and then reach
__isolate_free_page() and meanwhile watermarks changed so we wouldn't pass
without ALLOC_CMA anymore, it should be rare hopefully and not cause us deplete
non-CMA free pages too badly.

But I've only now also realized how dynamic setting cc->cma is. So in case a
zone consists mostly of CMA blocks, removing ALLOC_CMA in
__compaction_suitable() would be actually wrong and prevent compaction from
doing any work? Sigh. Any idea about that?

>>
>> And long-term what happens if the "CMA using ZONE_MOVABLE" approach is merged
>> and there are not more CMA migratetypes to test? Might this change actually also
>> avoid your issue, as said pages without __GFP_MOVABLE won't end up in a
>> ZONE_MOVABLE?
>
> Yeah, this is what I was thinking about. Basically I want to mimic this behavior
> right now. Once this approach will be implemented and merged, it will happen
> automatically: obviously, compaction won't move pages between different zones.

That will be much better. Can't wait, then :)

> Thank you!
>
> --
>
> From f4a0cfff41c7acab78116478e8e69ae8773b405c Mon Sep 17 00:00:00 2001
> From: Roman Gushchin <guro@fb.com>
> Date: Wed, 8 Apr 2020 11:16:38 -0700
> Subject: [PATCH v2] mm: compaction: avoid migrating non-cma pages to a cma
> area
>
> Compaction does treat cma pageblocks on pair with any movable
> pageblocks. It means it can easily move non-cma pages into a cma zone.
>
> It can create problems for the cma allocator.
>
> The particular problem I'm looking at is related to btrfs metadata
> pages, which are allocated without __GFP_MOVABLE, but beside that
> are generic pagecache pages. In fact, they are sometimes movable
> and sometimes not, depending on whether they are dirty and also
> on the extent buffer reference counter.
>
> Compaction moves them to the hugetlb_cma area, and then sometimes
> the cma allocator fails to move them back from the cma area. It
> results in failures of gigantic hugepages allocations.
>
> Also in general cma areas are reserved close to the end of a zone,
> and it's where compaction tries to migrate pages. It means
> compaction will aggressively fill cma areas, which makes not much
> sense.
>
> So to avoid it, let's preserve non-cma pages from being moved into
> a cma area. Because cma areas are usually quite large and the number
> of areas is small, it should not significantly affect the memory
> fragmentation.
>
> v2:
> 1) adjusted the __zone_watermark_ok() check, suggested by
> Vlastimil Babka
> 2) fixed a typo in a comment
>
> Signed-off-by: Roman Gushchin <guro@fb.com>>
> ---
> mm/compaction.c | 10 ++++++++--
> mm/internal.h | 1 +
> 2 files changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 46f0fcc93081..9ca036cb148a 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1159,6 +1159,10 @@ static bool suitable_migration_target(struct compact_control *cc,
> return false;
> }
>
> + /* Do not bring non-cma pages into a cma area */
> + if (is_migrate_cma(get_pageblock_migratetype(page)) && !cc->cma)

Nit: probably reverse this as the second test is cheaper?

> + return false;
> +
> if (cc->ignore_block_suitable)
> return true;
>