linux-kernel - Re: [PATCH 1/2] mm: Retry migration earlier upon refcount mismatch

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Message-ID: <cc1ababf-cda5-4eaf-9e67-47c26d5d70fe@arm.com>
Date: Mon, 12 Aug 2024 11:43:43 +0530
From: Dev Jain <dev.jain@....com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: akpm@...ux-foundation.org, shuah@...nel.org, david@...hat.com,
 willy@...radead.org, ryan.roberts@....com, anshuman.khandual@....com,
 catalin.marinas@....com, cl@...two.org, vbabka@...e.cz, mhocko@...e.com,
 apopple@...dia.com, osalvador@...e.de, baolin.wang@...ux.alibaba.com,
 dave.hansen@...ux.intel.com, will@...nel.org, baohua@...nel.org,
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 kirill.shutemov@...ux.intel.com, hughd@...gle.com, aneesh.kumar@...nel.org,
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 linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: Retry migration earlier upon refcount mismatch


On 8/12/24 11:04, Huang, Ying wrote:
> Hi, Dev,
>
> Dev Jain <dev.jain@....com> writes:
>
>> As already being done in __migrate_folio(), wherein we backoff if the
>> folio refcount is wrong, make this check during the unmapping phase, upon
>> the failure of which, the original state of the PTEs will be restored and
>> the folio lock will be dropped via migrate_folio_undo_src(), any racing
>> thread will make progress and migration will be retried.
>>
>> Signed-off-by: Dev Jain <dev.jain@....com>
>> ---
>>   mm/migrate.c | 9 +++++++++
>>   1 file changed, 9 insertions(+)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index e7296c0fb5d5..477acf996951 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1250,6 +1250,15 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
>>   	}
>>   
>>   	if (!folio_mapped(src)) {
>> +		/*
>> +		 * Someone may have changed the refcount and maybe sleeping
>> +		 * on the folio lock. In case of refcount mismatch, bail out,
>> +		 * let the system make progress and retry.
>> +		 */
>> +		struct address_space *mapping = folio_mapping(src);
>> +
>> +		if (folio_ref_count(src) != folio_expected_refs(mapping, src))
>> +			goto out;
>>   		__migrate_folio_record(dst, old_page_state, anon_vma);
>>   		return MIGRATEPAGE_UNMAP;
>>   	}
> Do you have some test results for this?  For example, after applying the
> patch, the migration success rate increased XX%, etc.

Noting that the migration selftest is operating on a single page,
before the patch, the test fails on shared-anon mappings on an
average of 10 iterations of move_pages(), and after applying the
patch it fails on average of 100 iterations, which makes sense
because the unmapping() will get retried 3 + 7 = 10 times.

>
> My understanding for this issue is that the migration success rate can
> increase if we undo all changes before retrying.  This is the current
> behavior for sync migration, but not for async migration.  If so, we can
> use migrate_pages_sync() for async migration too to increase success
> rate?  Of course, we need to change the function name and comments.
>
> --
> Best Regards,
> Huang, Ying