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

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Date:   Wed, 17 May 2023 16:33:51 -0600
From:   Khalid Aziz <khalid.aziz@...cle.com>
To:     David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org
Cc:     willy@...radead.org, steven.sistare@...cle.com,
        ying.huang@...el.com, mgorman@...hsingularity.net,
        khalid@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm, compaction: Skip all non-migratable pages during
 scan

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