Messages in this thread | | | Subject | Re: [PATCH] mm: migrate: handle freed page at the first place | From | Yang Shi <> | Date | Mon, 11 Nov 2019 16:02:30 -0800 |
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On 11/11/19 3:18 PM, Andrew Morton wrote: > On Tue, 12 Nov 2019 06:09:25 +0800 Yang Shi <yang.shi@linux.alibaba.com> 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. >> >> >> --- 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) { > Is it possible to have (!thp_migration_supported() && > PageTransHuge(page) && page_count(page) == 1)? If so, isn't this new > behviour?
IMHO it should be possible on some architectures, i.e. aarch64, with anonymous THP. I just saw PowerPC and x86_64 have CONFIG_ARCH_ENABLE_THP_MIGRATION selected. I'm not quite sure if I miss something.
It should be not new behavior since migrate_pages() should just split the THP then retry with base pages one by one. Even though it returns -EBUSY due to THP split failure in the current code, the behavior sounds problematic. We should not return errno for a freed page, right?
> >> /* 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);
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