linux-kernel - Re: [PATCH 1/3] mm: page_alloc: don't steal single pages from biggest buddy

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Message-ID: <68ac1af7-988b-42c6-8249-8949eb7fd986@suse.cz>
Date: Tue, 25 Feb 2025 11:50:02 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Johannes Weiner <hannes@...xchg.org>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Brendan Jackman <jackmanb@...gle.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] mm: page_alloc: don't steal single pages from biggest
 buddy

On 2/25/25 01:08, Johannes Weiner wrote:
> The fallback code searches for the biggest buddy first in an attempt
> to steal the whole block and encourage type grouping down the line.
> 
> The approach used to be this:
> 
> - Non-movable requests will split the largest buddy and steal the
>   remainder. This splits up contiguity, but it allows subsequent
>   requests of this type to fall back into adjacent space.
> 
> - Movable requests go and look for the smallest buddy instead. The
>   thinking is that movable requests can be compacted, so grouping is
>   less important than retaining contiguity.
> 
> c0cd6f557b90 ("mm: page_alloc: fix freelist movement during block
> conversion") enforces freelist type hygiene, which restricts stealing
> to either claiming the whole block or just taking the requested chunk;
> no additional pages or buddy remainders can be stolen any more.
> 
> The patch mishandled when to switch to finding the smallest buddy in
> that new reality. As a result, it may steal the exact request size,
> but from the biggest buddy. This causes fracturing for no good reason.
> 
> Fix this by committing to the new behavior: either steal the whole
> block, or fall back to the smallest buddy.
> 
> Remove single-page stealing from steal_suitable_fallback(). Rename it
> to try_to_steal_block() to make the intentions clear. If this fails,
> always fall back to the smallest buddy.
> 
> The following is from 4 runs of mmtest's thpchallenge. "Pollute" is
> single page fallback, "steal" is conversion of a partially used block.
> The numbers for free block conversions (omitted) are comparable.
> 
> 				     vanilla	      patched
> 
> @pollute[unmovable from reclaimable]:	  27		  106
> @pollute[unmovable from movable]:	  82		   46
> @pollute[reclaimable from unmovable]:	 256		   83
> @pollute[reclaimable from movable]:	  46		    8
> @pollute[movable from unmovable]:	4841		  868
> @pollute[movable from reclaimable]:	5278		12568
> 
> @steal[unmovable from reclaimable]:	  11		   12
> @steal[unmovable from movable]:		 113		   49
> @steal[reclaimable from unmovable]:	  19		   34
> @steal[reclaimable from movable]:	  47		   21
> @steal[movable from unmovable]:		 250		  183
> @steal[movable from reclaimable]:	  81		   93
> 
> The allocator appears to do a better job at keeping stealing and
> polluting to the first fallback preference. As a result, the numbers
> for "from movable" - the least preferred fallback option, and most
> detrimental to compactability - are down across the board.
> 
> Fixes: c0cd6f557b90 ("mm: page_alloc: fix freelist movement during block conversion")
> Suggested-by: Vlastimil Babka <vbabka@...e.cz>
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>

Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

Thanks!