Messages in this thread | | | Date | Wed, 29 Oct 2014 14:51:11 +0100 | From | Vlastimil Babka <> | Subject | Re: [PATCH 1/5] mm, compaction: pass classzone_idx and alloc_flags to watermark checking |
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On 10/28/2014 08:16 AM, Joonsoo Kim wrote:> On Mon, Oct 27, 2014 at 10:11:31AM +0100, Vlastimil Babka wrote: >> On 10/27/2014 07:46 AM, Joonsoo Kim wrote: >>> On Tue, Oct 07, 2014 at 05:33:35PM +0200, Vlastimil Babka wrote: >>> >>> Hello, >>> >>> compaction_suitable() has one more zone_watermark_ok(). Why is it >>> unchanged? >> >> Hi, >> >> it's a check whether there are enough free pages to perform compaction, >> which means enough migration targets and temporary copies during >> migration. These allocations are not affected by the flags of the >> process that makes the high-order allocation. > > Hmm... > > To check whether enough free page is there or not needs zone index and > alloc flag. What we need to ignore is just order information, IMO. > If there is not enough free page in that zone, compaction progress > doesn't have any meaning. It will fail due to shortage of free page > after successful compaction.
I thought that the second check in compaction_suitable() makes sure of this, but now I see it's in fact not. But i'm not sure if we should just put the flags in the first check, as IMHO the flags should only affect the final high-order allocation, not also the temporary pages needed for migration?
BTW now I'm not even sure that the 2UL << order part makes sense anymore. The number of pages migrated at once is always restricted by COMPACT_CLUSTER_MAX, so why would we need more than that to cover migration? Also the order of checks seems wrong. It should return COMPACT_PARTIAL "If the allocation would succeed without compaction" but that only can happen after passing the check if the zone has the extra 1UL << order for migration. Do you agree?
> I guess that __isolate_free_page() is also good candidate to need this > information in order to prevent compaction from isolating too many > freepage in low memory condition.
I don't see how it would help here. It's temporary allocations for page migration. How would passing classzone_idx and alloc_flags prevent isolating too many?
> Thanks. >
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