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@xxxxxxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
--
Mel Gorman
SUSE Labs
