linux-kernel - Re: [PATCH 4/8] mm: page_alloc: fix up block types when merging compatible blocks

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Message-ID: <20230821212018.GB106501@cmpxchg.org>
Date:   Mon, 21 Aug 2023 17:20:18 -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>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/8] mm: page_alloc: fix up block types when merging
 compatible blocks

On Mon, Aug 21, 2023 at 04:41:44PM -0400, Zi Yan wrote:
> On 21 Aug 2023, at 14:33, Johannes Weiner wrote:
> 
> > The buddy allocator coalesces compatible blocks during freeing, but it
> > doesn't update the types of the subblocks to match. When an allocation
> > later breaks the chunk down again, its pieces will be put on freelists
> > of the wrong type. This encourages incompatible page mixing (ask for
> > one type, get another), and thus long-term fragmentation.
> >
> > Update the subblocks when merging a larger chunk, such that a later
> > expand() will maintain freelist type hygiene.
> >
> > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> > ---
> >  mm/page_alloc.c | 37 ++++++++++++++++++++++---------------
> >  1 file changed, 22 insertions(+), 15 deletions(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index a5e36d186893..6c9f565b2613 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -438,6 +438,17 @@ void set_pageblock_migratetype(struct page *page, int migratetype)
> >  				page_to_pfn(page), MIGRATETYPE_MASK);
> >  }
> >
> > +static void change_pageblock_range(struct page *pageblock_page,
> > +					int start_order, int migratetype)
> > +{
> > +	int nr_pageblocks = 1 << (start_order - pageblock_order);
> > +
> > +	while (nr_pageblocks--) {
> > +		set_pageblock_migratetype(pageblock_page, migratetype);
> > +		pageblock_page += pageblock_nr_pages;
> > +	}
> > +}
> > +
> 
> Is this code move included by accident?

Ah, yes, my bad.

I used to call change_pageblock_range() at the end of the merge,
before adding the coalesced chunk to the freelist, for which I needed
this further up. Then I changed it to dealing with individual buddies
instead, and forgot to drop this part.

I'll remove it in the next version.