Re: [PATCH v5 4/4] mm/page_alloc: restrict max order of merging on isolated pageblock

[Vlastimil Babka]

On 11/03/2014 09:10 AM, Joonsoo Kim wrote:
> On Fri, Oct 31, 2014 at 03:39:13PM +0100, Vlastimil Babka wrote:
> > > +				__isolate_free_page(page, order);
> > > +				set_page_refcounted(page);
> > > +				isolated_page = page;
> > > +			}
> > > +		}
> > > +	}
> > >   	nr_pages = move_freepages_block(zone, page, migratetype);
> >
> > - this is a costly no-op when the whole pageblock is an isolated
> > page, right?
>
> Okay. I will fix it.
>
> > >   	__mod_zone_freepage_state(zone, nr_pages, migratetype);
> >
> > - with isolated_page set, this means you increase freepage_state
> > here, and then the second time in __free_pages() below?
> > __isolate_free_page() won't decrease it as the pageblock is still
> > MIGRATE_ISOLATE, so the net result is overcounting?
>
> After __isolate_free_page(), freepage has no buddy flag and
> move_freepages_block() doesn't move and count it. So, freepage_state
> only increase in __free_pages(). So net result will be correct.

Ah right, I forgot that it gets nr_pages from the move_freepages_block() 
result (which is 0 in this case).

> Below is the update for your comment.
>
> Thanks.
>
> ------------>8----------------
>  From 4bf298908aba16935c7699589c60d00fa0cf389c Mon Sep 17 00:00:00 2001
> From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Date: Mon, 25 Aug 2014 09:52:13 +0900
> Subject: [PATCH v6 4/4] mm/page_alloc: restrict max order of merging on isolated
>   pageblock
>
> Current pageblock isolation logic could isolate each pageblock
> individually. This causes freepage accounting problem if freepage with
> pageblock order on isolate pageblock is merged with other freepage on
> normal pageblock. We can prevent merging by restricting max order of
> merging to pageblock order if freepage is on isolate pageblock.
>
> Side-effect of this change is that there could be non-merged buddy
> freepage even if finishing pageblock isolation, because undoing pageblock
> isolation is just to move freepage from isolate buddy list to normal buddy
> list rather than to consider merging. So, the patch also makes undoing
> pageblock isolation consider freepage merge. When un-isolation, freepage
> with more than pageblock order and it's buddy are checked. If they are
> on normal pageblock, instead of just moving, we isolate the freepage and
> free it in order to get merged.
>
> Changes from v5:
> Avoid costly move_freepages_block() if there is no freepage.
> Some cosmetic changes.
>
> Changes from v4:
> Consider merging on un-isolation process.
>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>


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