Re: [PATCH] mm/compaction: count pages and stop correctly during page isolation.

[Yang Shi]

On Thu, Oct 29, 2020 at 1:04 PM Zi Yan <zi.yan@xxxxxxxx> wrote:
>
> From: Zi Yan <ziy@xxxxxxxxxx>
>
> In isolate_migratepages_block, when cc->alloc_contig is true, we are
> able to isolate compound pages, nr_migratepages and nr_isolated did not
> count compound pages correctly, causing us to isolate more pages than we
> thought. Use thp_nr_pages to count pages. Otherwise, we might be trapped
> in too_many_isolated while loop, since the actual isolated pages can go
> up to COMPACT_CLUSTER_MAX*512=16384, where COMPACT_CLUSTER_MAX is 32,

Is it that easy to run into? 16384 doesn't seem like too many pages, just 64MB.

> since we stop isolation after cc->nr_migratepages reaches to
> COMPACT_CLUSTER_MAX.
>
> In addition, after we fix the issue above, cc->nr_migratepages could
> never be equal to COMPACT_CLUSTER_MAX if compound pages are isolated,
> thus page isolation could not stop as we intended. Change the isolation
> stop condition to >=.

The fix looks sane to me. Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>

Shall you add Fixes tag to commit
1da2f328fa643bd72197dfed0c655148af31e4eb? And may cc stable.

>
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> ---
>  mm/compaction.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index ee1f8439369e..0683a4999581 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1012,8 +1012,8 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>
>  isolate_success:
>                 list_add(&page->lru, &cc->migratepages);
> -               cc->nr_migratepages++;
> -               nr_isolated++;
> +               cc->nr_migratepages += thp_nr_pages(page);
> +               nr_isolated += thp_nr_pages(page);
>
>                 /*
>                  * Avoid isolating too much unless this block is being
> @@ -1021,7 +1021,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>                  * or a lock is contended. For contention, isolate quickly to
>                  * potentially remove one source of contention.
>                  */
> -               if (cc->nr_migratepages == COMPACT_CLUSTER_MAX &&
> +               if (cc->nr_migratepages >= COMPACT_CLUSTER_MAX &&
>                     !cc->rescan && !cc->contended) {
>                         ++low_pfn;
>                         break;
> @@ -1132,7 +1132,7 @@ isolate_migratepages_range(struct compact_control *cc, unsigned long start_pfn,
>                 if (!pfn)
>                         break;
>
> -               if (cc->nr_migratepages == COMPACT_CLUSTER_MAX)
> +               if (cc->nr_migratepages >= COMPACT_CLUSTER_MAX)
>                         break;
>         }
>
> --
> 2.28.0
>
>