Messages in this thread | | | Subject | Re: [PATCH] mm: migrate: handle freed page at the first place | From | Yang Shi <> | Date | Tue, 12 Nov 2019 10:29:46 -0800 |
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On 11/12/19 12:04 AM, Michal Hocko wrote: > On Tue 12-11-19 06:09:25, 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 did two things before handling freed page: >> >> 1. Return -ENOMEM if the page is THP and THP migration is not supported. >> 2. Allocate target page unconditionally. >> >> Both makes not too much sense. If we handle freed page at the first place >> we don't have to worry about allocating/freeing target page and split >> THP at all. >> >> For example (worst case) if we are trying to migrate a freed THP without >> THP migration supported, the migrate_pages() would just split the THP then >> retry to migrate base pages one by one by pointless allocating and freeing >> pages, this is just waste of time. >> >> I didn't run into any actual problem with the current code (or I may >> just not notice it yet), it was found by visual inspection. > It would be preferable to accompany a change like this with some actual > numbers. A race with page freeing should be a very rare situation. Maybe > it is not under some workloads but that would better be checked and > documented. I also do not like to do page state changes for THP > migration without a support. I cannot really say this is 100% correct
However that THP will not be migrated actually, just will be freed by put_page().
> from top of my head and I do not see a sufficient justification to go > and chase all those tiny details because that is time consuming. > >> 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> >> --- >> mm/migrate.c | 18 +++++++----------- >> 1 file changed, 7 insertions(+), 11 deletions(-) >> >> diff --git a/mm/migrate.c b/mm/migrate.c >> index 4fe45d1..ef96997 100644 >> --- a/mm/migrate.c >> +++ b/mm/migrate.c >> @@ -1170,13 +1170,6 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page, >> int rc = MIGRATEPAGE_SUCCESS; >> struct page *newpage; >> >> - 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 +1180,16 @@ 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; >> } >> >> + if (!thp_migration_supported() && PageTransHuge(page)) >> + return -ENOMEM; >> + >> + 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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