linux-kernel - Re: [PATCH 10/10] mm: page_alloc: consolidate free page accounting

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Message-ID: <20240327143236.GE229434@cmpxchg.org>
Date: Wed, 27 Mar 2024 10:32:36 -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>, Zi Yan <ziy@...dia.com>,
	"Huang, Ying" <ying.huang@...el.com>,
	David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/10] mm: page_alloc: consolidate free page accounting

On Wed, Mar 27, 2024 at 09:54:01AM +0100, Vlastimil Babka wrote:
> On 3/20/24 7:02 PM, Johannes Weiner wrote:
> > Free page accounting currently happens a bit too high up the call
> > stack, where it has to deal with guard pages, compaction capturing,
> > block stealing and even page isolation. This is subtle and fragile,
> > and makes it difficult to hack on the code.
> > 
> > Now that type violations on the freelists have been fixed, push the
> > accounting down to where pages enter and leave the freelist.
> 
> Awesome!
> 
> > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> 
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
> 
> Just some nits:
> 
> > @@ -1314,10 +1349,10 @@ static inline void expand(struct zone *zone, struct page *page,
> >  		 * Corresponding page table entries will not be touched,
> >  		 * pages will stay not present in virtual address space
> >  		 */
> > -		if (set_page_guard(zone, &page[size], high, migratetype))
> > +		if (set_page_guard(zone, &page[size], high))
> >  			continue;
> >  
> > -		add_to_free_list(&page[size], zone, high, migratetype);
> > +		add_to_free_list(&page[size], zone, high, migratetype, false);
> 
> This is account_freepages() in the hot loop, what if we instead used
> __add_to_free_list(), sum up nr_pages and called account_freepages() once
> outside of the loop?

Good idea. I'll send a fixlet for that.

> >  		set_buddy_order(&page[size], high);
> >  	}
> >  }
> 
> <snip>
> 
> > diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> > index 042937d5abe4..914a71c580d8 100644
> > --- a/mm/page_isolation.c
> > +++ b/mm/page_isolation.c
> > @@ -252,7 +252,8 @@ static void unset_migratetype_isolate(struct page *page, int migratetype)
> >  		 * Isolating this block already succeeded, so this
> >  		 * should not fail on zone boundaries.
> >  		 */
> > -		WARN_ON_ONCE(!move_freepages_block_isolate(zone, page, migratetype));
> > +		WARN_ON_ONCE(!move_freepages_block_isolate(zone, page,
> > +							   migratetype));
> >  	} else {
> >  		set_pageblock_migratetype(page, migratetype);
> >  		__putback_isolated_page(page, order, migratetype);
> 
> Looks like a drive-by edit of an extra file just to adjust identation.

Argh, yeah, I think an earlier version mucked with the signature and I
didn't undo that cleanly. I'll send a fixlet for that too.

Thanks for the review!