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

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Message-ID: <42b050da-38b1-2244-6a9a-0320d5e7d2c4@oracle.com>
Date:   Tue, 23 May 2023 14:54:30 -0600
From:   Khalid Aziz <khalid.aziz@...cle.com>
To:     Baolin Wang <baolin.wang@...ux.alibaba.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 5/22/23 21:42, Baolin Wang wrote:
> 
> 
> On 5/18/2023 12:15 AM, 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.
>>
>> Signed-off-by: Khalid Aziz <khalid.aziz@...cle.com>
>> Suggested-by: Steve Sistare <steven.sistare@...cle.com>
>> ---
>> v3:
>>     - Account for extra ref added by get_page_unless_zero() earlier
>>       in isolate_migratepages_block() (Suggested by Huang, Ying)
>>     - Clean up computation of extra refs to be consistent
>>       (Suggested by Huang, Ying)
>>
>> v2:
>>     - Update comments in the code (Suggested by Andrew)
>>     - Use PagePrivate() instead of page_has_private() (Suggested
>>       by Matthew)
>>     - Pass mapping to page_has_extrarefs() (Suggested by Matthew)
>>     - Use page_ref_count() (Suggested by Matthew)
>>     - Rename is_pinned_page() to reflect its function more
>>       accurately (Suggested by Matthew)
>>
>>   mm/compaction.c | 36 ++++++++++++++++++++++++++++++++----
>>   1 file changed, 32 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 5a9501e0ae01..f04c00981172 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -764,6 +764,34 @@ static bool too_many_isolated(pg_data_t *pgdat)
>>       return too_many;
>>   }
>> +/*
>> + * Check if this base page should be skipped from isolation because
>> + * it has extra refcounts that will prevent it from being migrated.
>> + * This function is called for regular pages only, and not
>> + * for THP or hugetlbfs pages. This code is inspired by similar code
>> + * in migrate_vma_check_page(), can_split_folio() and
>> + * folio_migrate_mapping()
>> + */
>> +static inline bool page_has_extra_refs(struct page *page)
>> +{
>> +    /* caller holds a ref already from get_page_unless_zero() */
>> +    unsigned long extra_refs = 1;
>> +
>> +    /* anonymous page can have extra ref from swap cache */
>> +    if (PageAnon(page))
>> +        extra_refs += PageSwapCache(page) ? 1 : 0;
>> +    else
>> +        extra_refs += 1 + PagePrivate(page);
>> +
>> +    /*
>> +     * This is an admittedly racy check but good enough to determine
>> +     * if a page is pinned and can not be migrated
>> +     */
>> +    if ((page_ref_count(page) - extra_refs) > page_mapcount(page))
> 
> Could you explain why changing total_mapcount() to page_mapcount()? The original commit 829ae0f81ce0 ("mm: migrate: fix 
> THP's mapcount on isolation") did fix a THP's mapcount issue with converting to total_mapcount().

I had based this code on migrate_vma_check_page() which uses page_mapcount() but you are right. In the code for 
page_has_extra_refs(), this should stay total_mapcount(). I will fix it.

Thanks,
Khalid

> 
>> +        return true;
>> +    return false;
>> +}
>> +
>>   /**
>>    * isolate_migratepages_block() - isolate all migrate-able pages within
>>    *                  a single pageblock
>> @@ -992,12 +1020,12 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>>               goto isolate_fail;
>>           /*
>> -         * Migration will fail if an anonymous page is pinned in memory,
>> -         * so avoid taking lru_lock and isolating it unnecessarily in an
>> -         * admittedly racy check.
>> +         * Migration will fail if a page has extra refcounts
>> +         * preventing it from migrating, so avoid taking
>> +         * lru_lock and isolating it unnecessarily
>>            */
>>           mapping = page_mapping(page);
>> -        if (!mapping && (page_count(page) - 1) > total_mapcount(page))
>> +        if (page_has_extra_refs(page))
>>               goto isolate_fail_put;
>>           /*