Re: [PATCH v3] mm, compaction: Skip all non-migratable pages during scan

[Khalid Aziz]

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@xxxxxxxxxx>
> > Suggested-by: Steve Sistare <steven.sistare@xxxxxxxxxx>
> > ---
> > 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;
> >           /*