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

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Message-ID: <5f43a17c-94a0-4bff-b451-e5014de36ed9@redhat.com>
Date:   Fri, 19 May 2023 11:51:00 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Khalid Aziz <khalid.aziz@...cle.com>, akpm@...ux-foundation.org,
        willy@...radead.org, steven.sistare@...cle.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 18.05.23 03:09, Huang, Ying wrote:
> David Hildenbrand <david@...hat.com> 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