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

[Huang, Ying]

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.

> We don't want to fail early in case there is a short-term pin that
> might go away any moment after we isolated ... that will make the
> situation worse for these use cases, especially if MIGRATE_CMA or
> ZONE_MOVABLE is involved.

Best Regards,
Huang, Ying