Messages in this thread | | | Date | Tue, 23 May 2023 14:54:30 -0600 | Subject | Re: [PATCH v3] mm, compaction: Skip all non-migratable pages during scan | From | Khalid Aziz <> |
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On 5/22/23 21:42, Baolin Wang wrote: > > > On 5/18/2023 12:15 AM, Khalid Aziz wrote: >> 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 kcompactd scanning vfio pinned pages over and over again that can >> not be migrated. >> >> Signed-off-by: Khalid Aziz <khalid.aziz@oracle.com> >> Suggested-by: Steve Sistare <steven.sistare@oracle.com> >> --- >> v3: >> - Account for extra ref added by get_page_unless_zero() earlier >> in isolate_migratepages_block() (Suggested by Huang, Ying) >> - Clean up computation of extra refs to be consistent >> (Suggested by Huang, Ying) >> >> v2: >> - Update comments in the code (Suggested by Andrew) >> - Use PagePrivate() instead of page_has_private() (Suggested >> by Matthew) >> - Pass mapping to page_has_extrarefs() (Suggested by Matthew) >> - Use page_ref_count() (Suggested by Matthew) >> - Rename is_pinned_page() to reflect its function more >> accurately (Suggested by Matthew) >> >> mm/compaction.c | 36 ++++++++++++++++++++++++++++++++---- >> 1 file changed, 32 insertions(+), 4 deletions(-) >> >> diff --git a/mm/compaction.c b/mm/compaction.c >> index 5a9501e0ae01..f04c00981172 100644 >> --- a/mm/compaction.c >> +++ b/mm/compaction.c >> @@ -764,6 +764,34 @@ static bool too_many_isolated(pg_data_t *pgdat) >> 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_extra_refs(struct page *page) >> +{ >> + /* caller holds a ref already from get_page_unless_zero() */ >> + unsigned long extra_refs = 1; >> + >> + /* anonymous page can have extra ref from swap cache */ >> + if (PageAnon(page)) >> + extra_refs += PageSwapCache(page) ? 1 : 0; >> + else >> + extra_refs += 1 + PagePrivate(page); >> + >> + /* >> + * 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)) > > Could you explain why changing total_mapcount() to page_mapcount()? The original commit 829ae0f81ce0 ("mm: migrate: fix > THP's mapcount on isolation") did fix a THP's mapcount issue with converting to total_mapcount().
I had based this code on migrate_vma_check_page() which uses page_mapcount() but you are right. In the code for page_has_extra_refs(), this should stay total_mapcount(). I will fix it.
Thanks, Khalid
> >> + return true; >> + return false; >> +} >> + >> /** >> * isolate_migratepages_block() - isolate all migrate-able pages within >> * a single pageblock >> @@ -992,12 +1020,12 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn, >> 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_extra_refs(page)) >> goto isolate_fail_put; >> /*
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