Re: [PATCH 1/5] mm/compaction: allow blockpfn outside of pageblock for high order buddy page

From: Baolin Wang
Date: Tue Aug 01 2023 - 21:57:24 EST

On 7/30/2023 1:43 AM, Kemeng Shi wrote:
Commit 9fcd6d2e052ee ("mm, compaction: skip compound pages by order in
free scanner") skiped compound pages to save iterations and limit blockpfn
to reach outside of page block in case of bogus compound_order. While this
also limit pfnblock outside page block in case a buddy page with order
higher than page block is found. After this, isolate_freepages_range will
fail unexpectedly as it will fail to isolate the page block which was
isolated successfully by high order buddy page in previous page block
and abort the free page isolation.

Not sure I uderstand the problem, why the isolate_freepages_range() will fail? In isolate_freepages_range(), it did not use the 'blockpfn' cursor to try next candidate pfn, instead using the 'isolated' to calculate next cursor. Or I missed something else?

Fix this by allow blockpfn outside of pageblock in case of high order
buddy page.

Fixes: 9fcd6d2e052ee ("mm, compaction: skip compound pages by order in free scanner")
Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
mm/compaction.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 6841c0496223..d1d28d57e5bd 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -681,8 +681,10 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
* There is a tiny chance that we have read bogus compound_order(),
* so be careful to not go outside of the pageblock.
+ * Allow blockpfn outside pageblock in normal case that we isolate
+ * buddy page with order more than pageblock order.
- if (unlikely(blockpfn > end_pfn))
+ if (unlikely(blockpfn > end_pfn) && total_isolated <= pageblock_nr_pages)
blockpfn = end_pfn;
trace_mm_compaction_isolate_freepages(*start_pfn, blockpfn,
@@ -1418,7 +1420,7 @@ fast_isolate_around(struct compact_control *cc, unsigned long pfn)
isolate_freepages_block(cc, &start_pfn, end_pfn, &cc->freepages, 1, false);
/* Skip this pageblock in the future as it's full or nearly full */
- if (start_pfn == end_pfn && !cc->no_set_skip_hint)
+ if (start_pfn >= end_pfn && !cc->no_set_skip_hint)
@@ -1687,7 +1689,7 @@ static void isolate_freepages(struct compact_control *cc)
block_end_pfn, freelist, stride, false);
/* Update the skip hint if the full pageblock was scanned */
- if (isolate_start_pfn == block_end_pfn)
+ if (isolate_start_pfn >= block_end_pfn)
update_pageblock_skip(cc, page, block_start_pfn);
/* Are enough freepages isolated? */