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

[David Hildenbrand]

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).

--
Thanks,

David / dhildenb