linux-kernel - Re: [PATCH V2 0/6] mm: page_alloc: freelist migratetype hygiene

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Date:   Wed, 11 Oct 2023 11:45:32 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Zi Yan <ziy@...dia.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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 V2 0/6] mm: page_alloc: freelist migratetype hygiene

On Wed, Oct 11, 2023 at 11:25:27AM -0400, Johannes Weiner wrote:
> On Tue, Oct 10, 2023 at 05:12:01PM -0400, Johannes Weiner wrote:
> > On Mon, Oct 02, 2023 at 10:26:44PM -0400, Zi Yan wrote:
> > > @@ -1614,10 +1652,43 @@ static int move_freepages(struct zone *zone, unsigned long start_pfn,
> > >  
> > >  		order = buddy_order(page);
> > >  		move_to_free_list(page, zone, order, old_mt, new_mt);
> > > +		/*
> > > +		 * set page migratetype 1) only after we move all free pages in
> > > +		 * one pageblock and 2) for all pageblocks within the page.
> > > +		 *
> > > +		 * for 1), since move_to_free_list() checks page migratetype with
> > > +		 * old_mt and changing one page migratetype affects all pages
> > > +		 * within the same pageblock, if we are moving more than
> > > +		 * one free pages in the same pageblock, setting migratetype
> > > +		 * right after first move_to_free_list() triggers the warning
> > > +		 * in the following move_to_free_list().
> > > +		 *
> > > +		 * for 2), when a free page order is greater than pageblock_order,
> > > +		 * all pageblocks within the free page need to be changed after
> > > +		 * move_to_free_list().
> > 
> > I think this can be somewhat simplified.
> > 
> > There are two assumptions we can make. Buddies always consist of 2^n
> > pages. And buddies and pageblocks are naturally aligned. This means
> > that if this pageblock has the start of a buddy that straddles into
> > the next pageblock(s), it must be the first page in the block. That in
> > turn means we can move the handling before the loop.
> 
> Eh, scratch that. Obviously, a sub-block buddy can straddle blocks :(

I apologize for the back and forth, but I think I had it right the
first time. Say we have order-0 frees at pfn 511 and 512. Those can't
merge because their order-0 buddies are 510 and 513 respectively. The
same keeps higher-order merges below block size within the pageblock.
So again, due to the pow2 alignment, the only way for a buddy to
straddle a pageblock boundary is if it's >pageblock_order.

Please double check me on this, because I've stared at your patches
and the allocator code long enough now to thoroughly confuse myself.

My proposal for the follow-up changes still stands for now.