Re: [PATCH v2] mm, compaction: Skip all non-migratable pages during scan

[Khalid Aziz]

On 5/15/23 02:47, Huang, Ying wrote:
> Hi, Khalid,
>
> Cced Mel.
>
> Khalid Aziz <khalid.aziz@xxxxxxxxxx> writes:
>
> > 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
>
> s/kcompatd/kcompactd/

Thanks for the review. I will add the missing "c".

> > not be migrated.
>
> With the patch below, the cycles for kcompactd disappeared?

Yes, the test showed cpu time accumulated by kcompatcd during test run dropped significantly.

> > Signed-off-by: Khalid Aziz <khalid.aziz@xxxxxxxxxx>
> > Suggested-by: Steve Sistare <steven.sistare@xxxxxxxxxx>
> > Cc: Khalid Aziz <khalid@xxxxxxxxxx>
> > ---
> > 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..837f20df2bbb 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_extrarefs(struct page *page,
>
> Better to be named as page_has_extra_refs()?

Sure, I can do that.

> > +					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;
>
> IIUC, mapping != NULL if PageSwapCache(page) is true.  Please check the
> implementation of page_mapping().

which should work out for refcount since it would go to if part of conditional which will add a 1 for swap cache 
reference, but this code could be written better. I will work on it.

> And even if mapping == NULL, the extra_refs should be 1, because we have
> elevated the page refcount in isolate_migratepages_block() before
> checking whether the page is pinned.  IIUC, this is the original
> behavior.  Or, we can add "- 1" in the following checking.

You are right. There is a get_page_unless_zero() earlier in the code. I will compensate for that.

> > +
> > +	/*
> > +	 * 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
> > @@ -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_extrarefs(page, mapping))
> >   			goto isolate_fail_put;
> >   
> >   		/*
>
> Best Regards,
> Huang, Ying

Thanks,
Khalid