Re: [PATCH] mm: compaction: fix endless looping over same migrate block

[Vlastimil Babka]

On 7/31/23 19:24, 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""")

Yeah I suggested this commit for Fixes: as before the commit (or the
previous, reverted attempt) the skip would be set in
fast_find_migrateblock() so even though the issue of not handling unaligned
zones properly is older, it wouldn't cause an endless loop otherwise. Since
90ed667c03fe is rc1, we don't need stable.

> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  mm/compaction.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index dbc9f86b1934..eacca2794e47 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -912,11 +912,12 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  
>  		/*
>  		 * Check if the pageblock has already been marked skipped.
> -		 * Only the aligned PFN is checked as the caller isolates
> +		 * Only the first PFN is checked as the caller isolates
>  		 * COMPACT_CLUSTER_MAX at a time so the second call must
>  		 * not falsely conclude that the block should be skipped.
>  		 */
> -		if (!valid_page && pageblock_aligned(low_pfn)) {
> +		if (!valid_page && (pageblock_aligned(low_pfn) ||
> +				    low_pfn == cc->zone->zone_start_pfn)) {
>  			if (!isolation_suitable(cc, page)) {
>  				low_pfn = end_pfn;
>  				folio = NULL;
> @@ -2002,7 +2003,8 @@ static isolate_migrate_t isolate_migratepages(struct compact_control *cc)
>  		 * before making it "skip" so other compaction instances do
>  		 * not scan the same block.
>  		 */
> -		if (pageblock_aligned(low_pfn) &&
> +		if ((pageblock_aligned(low_pfn) ||
> +		     low_pfn == cc->zone->zone_start_pfn) &&
>  		    !fast_find_block && !isolation_suitable(cc, page))
>  			continue;
>