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Date:   Tue, 1 Aug 2023 10:08:12 +0100
From:   Mel Gorman <mgorman@techsingularity.net>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: compaction: fix endless looping over same migrate
 block
Message-ID: <20230801090812.3o6utiufbwhgevom@techsingularity.net>
References: <20230731172450.1632195-1-hannes@cmpxchg.org>
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On Mon, Jul 31, 2023 at 01:24:50PM -0400, Johannes Weiner wrote:
> During stress testing, the following situation was observed:
> 
>      70 root      39  19       0      0      0 R 100.0   0.0 959:29.92 khugepaged
>  310936 root      20   0   84416  25620    512 R  99.7   1.5 642:37.22 hugealloc
> 
> Tracing shows isolate_migratepages_block() endlessly looping over the
> first block in the DMA zone:
> 
>        hugealloc-310936  [001] ..... 237297.415718: mm_compaction_finished: node=0 zone=DMA      order=9 ret=no_suitable_page
>        hugealloc-310936  [001] ..... 237297.415718: mm_compaction_isolate_migratepages: range=(0x1 ~ 0x400) nr_scanned=513 nr_taken=0
>        hugealloc-310936  [001] ..... 237297.415718: mm_compaction_finished: node=0 zone=DMA      order=9 ret=no_suitable_page
>        hugealloc-310936  [001] ..... 237297.415718: mm_compaction_isolate_migratepages: range=(0x1 ~ 0x400) nr_scanned=513 nr_taken=0
>        hugealloc-310936  [001] ..... 237297.415718: mm_compaction_finished: node=0 zone=DMA      order=9 ret=no_suitable_page
>        hugealloc-310936  [001] ..... 237297.415718: mm_compaction_isolate_migratepages: range=(0x1 ~ 0x400) nr_scanned=513 nr_taken=0
>        hugealloc-310936  [001] ..... 237297.415718: mm_compaction_finished: node=0 zone=DMA      order=9 ret=no_suitable_page
>        hugealloc-310936  [001] ..... 237297.415718: mm_compaction_isolate_migratepages: range=(0x1 ~ 0x400) nr_scanned=513 nr_taken=0
> 
> The problem is that the functions tries to test and set the skip bit
> once on the block, to avoid skipping on its own skip-set, using
> pageblock_aligned() on the pfn as a test. But because this is the DMA
> zone which starts at pfn 1, this is never true for the first block,
> and the skip bit isn't set or tested at all. As a result,
> fast_find_migrateblock() returns the same pageblock over and over.
> 
> If the pfn isn't pageblock-aligned, also check if it's the start of
> the zone to ensure test-and-set-exactly-once on unaligned ranges.
> 
> Thanks to Vlastimil Babka for the help in debugging this.
> 
> Fixes: 90ed667c03fe ("Revert "Revert "mm/compaction: fix set skip in fast_find_migrateblock""")
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Acked-by: Mel Gorman <mgorman@techsingularity.net>

-- 
Mel Gorman
SUSE Labs