Re: [PATCH] mm, compaction: Skip all pinned pages during scan

[Khalid Aziz]

On 5/11/23 21:45, Andrew Morton wrote:
> On Thu, 11 May 2023 10:55:16 -0600 Khalid Aziz <khalid.aziz@xxxxxxxxxx> wrote:
>
> > Pinned pages can not be migrated. Currently as
> > isolate_migratepages_block() scans pages for compaction, it skips
> > any pinned anonymous pages. All pinned pages should be skipped and
> > not just the anonymous pinned pages. This patch adds a check for
> > pinned page by comparing its refcount with mapcount and accounts for
> > possible extra refcounts. 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.
> >
> > Signed-off-by: Khalid Aziz <khalid.aziz@xxxxxxxxxx>
> > Suggested-by: Steve Sistare <steven.sistare@xxxxxxxxxx>
> > ---
> >   mm/compaction.c | 33 +++++++++++++++++++++++++++++----
> >   1 file changed, 29 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/compaction.c b/mm/compaction.c
> > index 5a9501e0ae01..d1371fd75391 100644
> > --- a/mm/compaction.c
> > +++ b/mm/compaction.c
> > @@ -764,6 +764,32 @@ static bool too_many_isolated(pg_data_t *pgdat)
> >   	return too_many;
> >   }
> >   
> > +/*
> > + * Check if this base page should be skipped from isolation because
> > + * it is pinned. 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 is_pinned_page(struct page *page)
> > +{
> > +	unsigned long extra_refs;
> > +
> > +	/* anonymous page can have extra ref from page cache */
>
> "from swapcache"?
>
> > +	if (page_mapping(page))
> > +		extra_refs = 1 + page_has_private(page);
> > +	else
> > +		extra_refs = PageSwapCache(page) ? 1 : 0;
> > +
> > +	/*
> > +	 * This is an admittedly racy check but good enough to determine
> > +	 * if a page should be isolated
>
> "cannot be migrated"?

Thanks, Andrew! I will fix these and send out v2 patch.

--
Khalid