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

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Date:   Mon, 22 May 2023 09:12:38 -0600
From:   Khalid Aziz <khalid.aziz@...cle.com>
To:     "Huang, Ying" <ying.huang@...el.com>,
        David Hildenbrand <david@...hat.com>
Cc:     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 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;

                 /*


Thanks,
Khalid