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SubjectRe: [v2 PATCH] mm: migrate: handle freed page at the first place
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On 11/14/19 10:56 AM, Michal Hocko wrote:
> On Fri 15-11-19 02:24:29, Yang Shi wrote:
>> When doing migration if the freed page is met, we just return without
>> migrating it since it is pointless to migrate a freed page. But, the
>> current code allocates target page unconditionally before handling freed
>> page, if the page is freed, the newly allocated will be just freed. It
>> doesn't make too much sense and is just a waste of time although
>> migrating freed page is rare.
>>
>> So, handle freed page at the before that to avoid unnecessary page
>> allocation and free.
>>
>> Cc: Michal Hocko <mhocko@suse.com>
>> Cc: Mel Gorman <mgorman@techsingularity.net>
>> Cc: Vlastimil Babka <vbabka@suse.cz>
>> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
> I would be really surprised if this led to any runtime visible effect
> but I do agree that one less put_page path looks slightly better. For
> that reason
> Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!

>
>> ---
>> v2: * Keep thp migration support check before handling freed page per Michal Hocko
>> * Fixed the build warning reported by 0-day
>>
>> mm/migrate.c | 14 +++++---------
>> 1 file changed, 5 insertions(+), 9 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 4fe45d1..a8f87cb 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1168,15 +1168,11 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
>> enum migrate_reason reason)
>> {
>> int rc = MIGRATEPAGE_SUCCESS;
>> - struct page *newpage;
>> + struct page *newpage = NULL;
>>
>> if (!thp_migration_supported() && PageTransHuge(page))
>> return -ENOMEM;
>>
>> - newpage = get_new_page(page, private);
>> - if (!newpage)
>> - return -ENOMEM;
>> -
>> if (page_count(page) == 1) {
>> /* page was freed from under us. So we are done. */
>> ClearPageActive(page);
>> @@ -1187,13 +1183,13 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
>> __ClearPageIsolated(page);
>> unlock_page(page);
>> }
>> - if (put_new_page)
>> - put_new_page(newpage, private);
>> - else
>> - put_page(newpage);
>> goto out;
>> }
>>
>> + newpage = get_new_page(page, private);
>> + if (!newpage)
>> + return -ENOMEM;
>> +
>> rc = __unmap_and_move(page, newpage, force, mode);
>> if (rc == MIGRATEPAGE_SUCCESS)
>> set_page_owner_migrate_reason(newpage, reason);
>> --
>> 1.8.3.1
>>

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