linux-kernel - Re: [PATCH 3/6] mm: page_alloc: move free pages when converting block during isolation

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Date:   Mon, 11 Sep 2023 16:47:51 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Zi Yan <ziy@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] mm: page_alloc: move free pages when converting
 block during isolation

On Mon, Sep 11, 2023 at 04:17:07PM -0400, Zi Yan wrote:
> On 11 Sep 2023, at 15:41, Johannes Weiner wrote:
> 
> > When claiming a block during compaction isolation, move any remaining
> > free pages to the correct freelists as well, instead of stranding them
> > on the wrong list. Otherwise, this encourages incompatible page mixing
> > down the line, and thus long-term fragmentation.
> >
> > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> > ---
> >  mm/page_alloc.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 3db405414174..f6f658c3d394 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -2548,9 +2548,12 @@ int __isolate_free_page(struct page *page, unsigned int order)
> >  			 * Only change normal pageblocks (i.e., they can merge
> >  			 * with others)
> >  			 */
> > -			if (migratetype_is_mergeable(mt))
> > +			if (migratetype_is_mergeable(mt)) {
> >  				set_pageblock_migratetype(page,
> >  							  MIGRATE_MOVABLE);
> > +				move_freepages_block(zone, page,
> > +						     MIGRATE_MOVABLE, NULL);
> > +			}
> >  		}
> >  	}
> >
> > -- 
> > 2.42.0
> 
> Is this needed?

Yes, the problem is if we e.g. isolate half a block, then we'll
convert the type of the whole block but strand the half we're not
isolating. This can be a couple of hundred pages on the wrong list.

> And is this correct?
> 
> __isolate_free_page() removes the free page from a free list, but the added
> move_freepages_block() puts the page back to another free list, making
> __isolate_free_page() not do its work. OK. the for loop is going through
> the pages within the pageblock, so move_freepages_block() should be used
> on the rest of the pages on the pageblock.
> 
> So to make this correct, the easies change might be move
> del_page_from_free_list(page, zone, order) below this code chunk.

There is a del_page_from_freelist() just above this diff hunk. That
takes the page off the list and clears its PageBuddy.

move_freepages_block() will then move only the remainder of the block
that's still on the freelist with a mismatched type (move_freepages()
only moves buddies).