Re: [PATCH] cma: cached pageblock type fixup

[Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>]

On Tuesday 19 June 2012 11:00:15 Mel Gorman wrote:
> On Wed, May 23, 2012 at 09:22:00AM +0200, Bartlomiej Zolnierkiewicz wrote:
> > From: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
> > Subject: [PATCH] cma: cached pageblock type fixup
> > 
> > CMA pages added to per-cpu pages lists in free_hot_cold_page()
> > have private field set to MIGRATE_CMA pageblock type .  If this
> > happes just before start_isolate_page_range() in alloc_contig_range()
> > changes pageblock type of the page to MIGRATE_ISOLATE it may result
> > in the cached pageblock type being stale in free_pcppages_bulk()
> > (which may be triggered by drain_all_pages() in alloc_contig_range()),
> 
> So what?

"page being added to MIGRATE_CMA free list instead of MIGRATE_ISOLATE
one in __free_one_page() and (if the page is reused just before
test_pages_isolated() check) causing alloc_contig_range() failure."

> The pages get freed to the MIGRATE_CMA region. At worst they will be
> used for an allocation request that is compatible with being migrated by
> CMA. This will delay the allocation time of alloc_contig_range() but is
> hardly critical.

There is no waiting on MIGRATE_CMA pages in alloc_contig_range() to
become free again.  If the conversion to MIGRATE_ISOLATE fails then
the whole alloc_contig_range() allocation fails on test_pages_isolated()
check.

> Your fix on the other hand adds another call to get_pageblock_type() to
> free_pcppages_bulk which is expensive. The change made to
> buffered_rmqueue() is horrific. It takes the per-cpu page allocation
> path and adds a spin lock to it which completely defeats the purpose of
> having the per-cpu allocation avoid taking locks. This will have a very
> heavy impact on performance, particularly on parallel workloads.
> 
> As the impact of the race should be marginal and the cost of the fix is
> so unbelivably high I'm nacking this patch. If this race is a problem then
> it should be handled in alloc_contig_range() not in the allocator fast paths.

Do you have any idea how this race can be handled in alloc_contig_range()?

Best regards,
--
Bartlomiej Zolnierkiewicz
Samsung Poland R&D Center

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