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

[Khalid Aziz]

On 5/17/23 12:32, David Hildenbrand wrote:
> 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().
>
> 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.

You are right that transitory conditions can be problematic. Wouldn't that apply to anonymous pages as well and we do 
skip pinned anonymous pages today? A retry would be the right way to handle transitory conditions I think. At the same 
time, by not scanning long term pinned non-anonymous pages repeatedly, alloc_contig_range() would be helped as well, right?

Nevertheless, we certainly do not want a change that makes overall system behavior worse. Do you see system behavior 
getting worse, or would the retry in cma_alloc() be sufficient to deal with transitory pins?

Thanks,
Khalid