On 29 Oct 2020, at 17:14, Yang Shi wrote:

On Thu, Oct 29, 2020 at 1:04 PM Zi Yan zi.yan@sent.com wrote:

...

From: Zi Yan ziy@nvidia.com

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.

I hit this when I was running oom01 from ltp to test my PUD THP patchset, which allocates PUD THPs from CMA regions and splits them into PMD THPs due to memory pressure. I am not sure if it is common that in the upstream kernel PMD THPs will be allocated in CMA regions due to allocation fallback.

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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@gmail.com

Thanks.

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

Sure.

Fixes: 1da2f328fa64 (“mm,thp,compaction,cma: allow THP migration for CMA allocations”)

stable cc’ed.

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Signed-off-by: Zi Yan ziy@nvidia.com

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

— Best Regards, Yan Zi