linux-kernel - Re: [PATCH 4/6] mm: page_alloc: fix move_freepages

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Message-ID: <20230914133748.GE48476@cmpxchg.org>
Date:   Thu, 14 Sep 2023 09:37:48 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Zi Yan <ziy@...dia.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/6] mm: page_alloc: fix move_freepages_block() range
 error

On Wed, Sep 13, 2023 at 04:40:48PM +0200, Vlastimil Babka wrote:
> On 9/11/23 21:41, Johannes Weiner wrote:
> > When a block is partially outside the zone of the cursor page, the
> > function cuts the range to the pivot page instead of the zone
> > start. This can leave large parts of the block behind, which
> > encourages incompatible page mixing down the line (ask for one type,
> > get another), and thus long-term fragmentation.
> > 
> > This triggers reliably on the first block in the DMA zone, whose
> > start_pfn is 1. The block is stolen, but everything before the pivot
> > page (which was often hundreds of pages) is left on the old list.
> > 
> > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> 
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

Thanks!

> > @@ -1652,7 +1652,7 @@ int move_freepages_block(struct zone *zone, struct page *page,
> >  
> >  	/* Do not cross zone boundaries */
> >  	if (!zone_spans_pfn(zone, start_pfn))
> > -		start_pfn = pfn;
> > +		start_pfn = zone->zone_start_pfn;
> >  	if (!zone_spans_pfn(zone, end_pfn))
> >  		return 0;
> 
> Culdn't we also adjust end_pfn to zone_end_pfn() so we don't just ignore the
> last half-pageblock for no good reason? (or am I missing any?)
> Also would stop treating end_pfn as inclusive here and in move_freepages(),
> it's rather uncommon.

You raise a good point here and in the reply to 5/6. Let me reply to
the other email.