linux-kernel - Re: [RFC PATCH 25/26] mm: page_alloc: disallow fallbacks when 2M defrag is enabled

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Message-ID: <20230421145657.fnpjqkuyquy3z24t@techsingularity.net>
Date:   Fri, 21 Apr 2023 15:56:57 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     linux-mm@...ck.org, Kaiyang Zhao <kaiyang2@...cmu.edu>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Rientjes <rientjes@...gle.com>,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [RFC PATCH 25/26] mm: page_alloc: disallow fallbacks when 2M
 defrag is enabled

On Tue, Apr 18, 2023 at 03:13:12PM -0400, Johannes Weiner wrote:
> Fallbacks are already unlikely due to watermarks being enforced
> against MIGRATE_FREE blocks. Eliminate them altogether. This allows
> compaction to look exclusively at movable blocks, reducing the number
> of pageblocks it needs to scan on an ongoing basis.
> 
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>

Conceptually this could be fun if a GFP_NOFS allocation cannot migrate
enough memory to free one pageblock and there are no pageblocks
available of the correct migratetype. Fallbacks might be unlikely but
never being able to fallback is a livelock risk, no?

-- 
Mel Gorman
SUSE Labs