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Message-ID: <47dca08e-a100-1099-27a9-7c1d92ffa51d@oracle.com>
Date: Fri, 12 May 2023 09:30:59 -0600
From: Khalid Aziz <khalid.aziz@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: willy@...radead.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org,
Steve Sistare <steven.sistare@...cle.com>
Subject: Re: [PATCH] mm, compaction: Skip all pinned pages during scan
On 5/11/23 21:45, Andrew Morton wrote:
> On Thu, 11 May 2023 10:55:16 -0600 Khalid Aziz <khalid.aziz@...cle.com> wrote:
>
>> Pinned pages can not be migrated. Currently as
>> isolate_migratepages_block() scans pages for compaction, it skips
>> any pinned anonymous pages. All pinned pages should be skipped and
>> not just the anonymous pinned pages. This patch adds a check for
>> pinned page by comparing its refcount with mapcount and accounts for
>> possible extra refcounts. 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 kcompatd 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>
>> ---
>> mm/compaction.c | 33 +++++++++++++++++++++++++++++----
>> 1 file changed, 29 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 5a9501e0ae01..d1371fd75391 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -764,6 +764,32 @@ static bool too_many_isolated(pg_data_t *pgdat)
>> return too_many;
>> }
>>
>> +/*
>> + * Check if this base page should be skipped from isolation because
>> + * it is pinned. 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 is_pinned_page(struct page *page)
>> +{
>> + unsigned long extra_refs;
>> +
>> + /* anonymous page can have extra ref from page cache */
>
> "from swapcache"?
>
>> + if (page_mapping(page))
>> + extra_refs = 1 + page_has_private(page);
>> + else
>> + extra_refs = PageSwapCache(page) ? 1 : 0;
>> +
>> + /*
>> + * This is an admittedly racy check but good enough to determine
>> + * if a page should be isolated
>
> "cannot be migrated"?
>
Thanks, Andrew! I will fix these and send out v2 patch.
--
Khalid
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