Re: [PATCH 03/10] mm/page_alloc: handle page on pcp correctly if it's pageblock is isolated

From: Joonsoo Kim
Date: Mon Jul 14 2014 - 02:18:37 EST

On Mon, Jul 07, 2014 at 05:19:48PM +0200, Vlastimil Babka wrote:
> On 07/04/2014 09:57 AM, Joonsoo Kim wrote:
> >If pageblock of page on pcp are isolated now, we should free it to isolate
> >buddy list to prevent future allocation on it. But current code doesn't
> >do this.
> >
> >Moreover, there is a freepage counting problem on current code. Although
> >pageblock of page on pcp are isolated now, it could go normal buddy list,
> >because get_onpcp_migratetype() will return non-isolate migratetype.
> get_onpcp_migratetype() is only introduced in later patch.

Yes, I will fix it.

> >In this case, we should do either adding freepage count or changing
> >migratetype to MIGRATE_ISOLATE, but, current code do neither.
> I wouldn't say it "do neither". It already limits the freepage
> counting to !MIGRATE_ISOLATE case (and it's not converted to
> __mod_zone_freepage_state for some reason). So there's accounting
> mismatch in addition to buddy list misplacement.


> >This patch fixes these two problems by handling pageblock migratetype
> >before calling __free_one_page(). And, if we find the page on isolated
> >pageblock, change migratetype to MIGRATE_ISOLATE to prevent future
> >allocation of this page and freepage counting problem.
> So although this is not an addition of a new pageblock migratetype
> check to the fast path (the check is already there), I would prefer
> removing the check :)

Yes, I want to do it if possible. :)

> With the approach of pcplists draining
> outlined in my reply to 00/10, we would allow a misplacement to
> happen (and the page accounted as freepage) immediately followed by
> move_frepages_block which would place the page onto isolate freelist
> with the rest. Anything newly freed will get isolate_migratetype
> determined in free_hot_cold_page or __free_pages_ok (where it would
> need moving the migratepage check under the disabled irq part) and
> be placed and buddy-merged properly.

I explained the problem of this approach in 00/10.


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