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

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Message-ID: <20231011152525.GA461170@cmpxchg.org>
Date:   Wed, 11 Oct 2023 11:25:25 -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 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 :(

So forget about my version of move_free_pages(). Only consider the
changes to find_straddling_buddy() and my question about multiple
blocks inside the requested range.

But I do have another question about your patch then. Say you have an
order-1 buddy that straddles into the block:

+       /* split at start_pfn if it is in the middle of a free page */
+       if (new_start_pfn != start_pfn && PageBuddy(pfn_to_page(new_start_pfn))) {
+               struct page *new_page = pfn_to_page(new_start_pfn);
+               int new_page_order = buddy_order(new_page);
+
+               if (new_start_pfn + (1 << new_page_order) > start_pfn) {
+                       /* change migratetype so that split_free_page can work */
+                       set_pageblock_migratetype(pfn_to_page(start_pfn), new_mt);
+                       split_free_page(new_page, buddy_order(new_page),
+                                       start_pfn - new_start_pfn);
+
+                       mt_changed_pfn = start_pfn;
+                       /* move to next page */
+                       start_pfn = new_start_pfn + (1 << new_page_order);
+               }
+       }

this will have changed the type of the block to new_mt.

But then the buddy scan will do this:

                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().

That move_to_free_list() will complain that the pages no longer match
old_mt, no?