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

[Huang, Ying]

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.

Best Regards,
Huang, Ying