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

[Khalid Aziz]

On 5/21/23 23:55, Huang, Ying wrote:
> David Hildenbrand <david@xxxxxxxxxx> writes:
>
> > On 18.05.23 03:09, Huang, Ying wrote:
> > > David Hildenbrand <david@xxxxxxxxxx> writes:
> > >
> > > > On 17.05.23 18:15, 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.
> > > >
> > > > How will this change affect alloc_contig_range(), such as used for CMA
> > > > allocations or virtio-mem? alloc_contig_range() ends up calling
> > > > isolate_migratepages_range() -> isolate_migratepages_block().
> > > IIUC, cc->alloc_contig can be used to distinguish contiguous
> > > allocation
> > > and compaction.  And, from the original commit which introduced
> > > anonymous pages skipping (commit 119d6d59dcc0 ("mm, compaction: avoid
> > > isolating pinned pages ")) and this patch, large number of migration
> > > failure during compaction causes real issue too.  So, I suggest to use
> > > cc->alloc_contig here.
> >
> > Agreed. I further wonder if we want to special-case the !alloc_contig
> > case also for MIGRATE_CMA and ZONE_MOVABLE, where we cannot have
> > longterm page pinnings (e.g., vfio pinned pages).
>
> This makes sense.  The skipping is more accurate in this way.


Something like this?

diff --git a/mm/compaction.c b/mm/compaction.c
index f04c00981172..014e21d3d7e9 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1025,7 +1025,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
                 * lru_lock and isolating it unnecessarily
                 */
                mapping = page_mapping(page);
-               if (page_has_extra_refs(page))
+               if (!cc->alloc_contig && page_has_extra_refs(page))
                        goto isolate_fail_put;

                /*


Thanks,
Khalid