The patch titled
Subject: mm, compaction: Skip all non-migratable pages during scan
has been added to the -mm mm-unstable branch. Its filename is
mm-compaction-skip-all-non-migratable-pages-during-scan.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-compaction-skip-all-non-migratable-pages-during-scan.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Khalid Aziz <khalid.aziz@xxxxxxxxxx>
Subject: mm, compaction: Skip all non-migratable pages during scan
Date: Fri, 12 May 2023 13:00:00 -0600
Pages pinned in memory through extra refcounts can not be migrated.
Currently as isolate_migratepages_block() scans pages for compaction, it
skips any pinned anonymous pages. All non-migratable pages should be
skipped and not just the anonymous pinned pages.
This patch adds a check for extra refcounts on a page to determine if the
page can be migrated. This was seen as a real issue on a customer
workload where a large number of pages were pinned by vfio on the host and
any attempts to allocate hugepages resulted in significant amount of cpu
time spent in either direct compaction or in kcompatd scanning vfio pinned
pages over and over again that can not be migrated.
Link: https://lkml.kernel.org/r/20230512190000.103737-1-khalid.aziz@xxxxxxxxxx
Signed-off-by: Khalid Aziz <khalid.aziz@xxxxxxxxxx>
Suggested-by: Steve Sistare <steven.sistare@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/compaction.c | 36 ++++++++++++++++++++++++++++++++----
1 file changed, 32 insertions(+), 4 deletions(-)
--- a/mm/compaction.c~mm-compaction-skip-all-non-migratable-pages-during-scan
+++ a/mm/compaction.c
@@ -765,6 +765,34 @@ static bool too_many_isolated(pg_data_t
return too_many;
}
+/*
+ * Check if this base page should be skipped from isolation because
+ * it has extra refcounts that will prevent it from being migrated.
+ * This function is called for regular pages only, and not
+ * for THP or hugetlbfs pages. This code is inspired by similar code
+ * in migrate_vma_check_page(), can_split_folio() and
+ * folio_migrate_mapping()
+ */
+static inline bool page_has_extrarefs(struct page *page,
+ struct address_space *mapping)
+{
+ unsigned long extra_refs;
+
+ /* anonymous page can have extra ref from swap cache */
+ if (mapping)
+ extra_refs = 1 + PagePrivate(page);
+ else
+ extra_refs = PageSwapCache(page) ? 1 : 0;
+
+ /*
+ * This is an admittedly racy check but good enough to determine
+ * if a page is pinned and can not be migrated
+ */
+ if ((page_ref_count(page) - extra_refs) > page_mapcount(page))
+ return true;
+ return false;
+}
+
/**
* isolate_migratepages_block() - isolate all migrate-able pages within
* a single pageblock
@@ -1003,12 +1031,12 @@ isolate_migratepages_block(struct compac
goto isolate_fail;
/*
- * Migration will fail if an anonymous page is pinned in memory,
- * so avoid taking lru_lock and isolating it unnecessarily in an
- * admittedly racy check.
+ * Migration will fail if a page has extra refcounts
+ * preventing it from migrating, so avoid taking
+ * lru_lock and isolating it unnecessarily
*/
mapping = page_mapping(page);
- if (!mapping && (page_count(page) - 1) > total_mapcount(page))
+ if (page_has_extrarefs(page, mapping))
goto isolate_fail_put;
/*
_
Patches currently in -mm which might be from khalid.aziz@xxxxxxxxxx are
mm-compaction-skip-all-non-migratable-pages-during-scan.patch
