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

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Date:   Tue, 23 May 2023 09:23:25 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Khalid Aziz <khalid.aziz@...cle.com>
Cc:     David Hildenbrand <david@...hat.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

Khalid Aziz <khalid.aziz@...cle.com> writes:

> On 5/21/23 23:55, Huang, Ying wrote:
>> David Hildenbrand <david@...hat.com> writes:
>> 
>>> 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).
>> This makes sense.  The skipping is more accurate in this way.
>> 
>
>
> Something like this?
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index f04c00981172..014e21d3d7e9 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1025,7 +1025,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>                  * lru_lock and isolating it unnecessarily
>                  */
>                 mapping = page_mapping(page);
> -               if (page_has_extra_refs(page))
> +               if (!cc->alloc_contig && page_has_extra_refs(page))
>                         goto isolate_fail_put;
>
>                 /*

As suggested by David above, you can check the current zone type (for
ZONE_MOVABLE) and page block migrate type (MIGRATE_CMA) too.  Because
pages there will not be pinned in long term, and should be tried to be
migrated.

Best Regards,
Huang, Ying