linux-kernel - Re: [PATCH v3 1/7] mm/compaction: skip useless pfn when updating cached pfn

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Date:	Mon, 7 Dec 2015 16:37:30 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Minchan Kim <minchan@...nel.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH v3 1/7] mm/compaction: skip useless pfn when updating
 cached pfn

On Thu, Dec 03, 2015 at 11:36:52AM +0100, Vlastimil Babka wrote:
> On 12/03/2015 08:11 AM, Joonsoo Kim wrote:
> > Cached pfn is used to determine the start position of scanner
> > at next compaction run. Current cached pfn points the skipped pageblock
> > so we uselessly checks whether pageblock is valid for compaction and
> > skip-bit is set or not. If we set scanner's cached pfn to next pfn of
> > skipped pageblock, we don't need to do this check.
> > 
> > This patch moved update_pageblock_skip() to
> > isolate_(freepages|migratepages). Updating pageblock skip information
> > isn't relevant to CMA so they are more appropriate place
> > to update this information.
> 
> That's step in a good direction, yeah. But why not go as far as some variant of
> my (not resubmitted) patch "mm, compaction: decouple updating pageblock_skip and
> cached pfn" [1]. Now the overloading of update_pageblock_skip() is just too much
> - a struct page pointer for the skip bits, and a pfn of different page for the
> cached pfn update, that's just more complex than it should be.
> 
> (I also suspect the pageblock_flags manipulation functions could be simpler if
> they accepted zone pointer and pfn instead of struct page)

Okay.

> Also recently in Aaron's report we found a possible scenario where pageblocks
> are being skipped without entering the isolate_*_block() functions, and it would
> make sense to update the cached pfn's in that case, independently of updating
> pageblock skip bits.
> 
> But this might be too out of scope of your series, so if you want I can
> separately look at reviving some useful parts of [1] and the simpler
> pageblock_flags manipulations.

I will cherry-pick some useful parts of that with your authorship and
respin this series after you finish to review all patches.

Thanks.

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