On Mon 19-11-18 13:51:21, Michal Hocko wrote: > On Mon 19-11-18 13:40:33, Michal Hocko wrote: > > On Mon 19-11-18 18:52:02, Baoquan He wrote: > > [...] > > > > There are few stacks directly in the offline path but those should be > > OK. > > The real culprit seems to be the swap in code > > > > > [ +1.734416] CPU: 255 PID: 5558 Comm: stress Tainted: G L 4.20.0-rc2+ #7 > > > [ +0.007927] Hardware name: 9008/IT91SMUB, BIOS BLXSV512 03/22/2018 > > > [ +0.006297] Call Trace: > > > [ +0.002537] dump_stack+0x46/0x60 > > > [ +0.003386] __migration_entry_wait.cold.65+0x5/0x14 > > > [ +0.005043] do_swap_page+0x84e/0x960 > > > [ +0.003727] ? arch_tlb_finish_mmu+0x29/0xc0 > > > [ +0.006412] __handle_mm_fault+0x933/0x1330 > > > [ +0.004265] handle_mm_fault+0xc4/0x250 > > > [ +0.003915] __do_page_fault+0x2b7/0x510 > > > [ +0.003990] do_page_fault+0x2c/0x110 > > > [ +0.003729] ? page_fault+0x8/0x30 > > > [ +0.003462] page_fault+0x1e/0x30 > > > > There are many traces to this path. We are > > /* > > * Once page cache replacement of page migration started, page_count > > * *must* be zero. And, we don't want to call wait_on_page_locked() > > * against a page without get_page(). > > * So, we use get_page_unless_zero(), here. Even failed, page fault > > * will occur again. > > */ > > if (!get_page_unless_zero(page)) > > goto out; > > pte_unmap_unlock(ptep, ptl); > > wait_on_page_locked(page); > > > > taking a reference to the page under the migration. I have to think > > about this much more but I suspec this is just calling for a problem. > > > > Cc migration experts. For you background information. We are seeing > > memory offline not being able to converge because few heavily used pages > > fail to migrate away - e.g. http://lkml.kernel.org/r/20181116012433.GU2653@MiWiFi-R3L-srv > > A debugging page to dump stack for these pages http://lkml.kernel.org/r/20181116091409.GD14706@xxxxxxxxxxxxxx > > shows that references are taken from the swap in code (above). How are > > we supposed to converge when the swapin code waits for the migration to > > finish with the reference count elevated? > > Just to clarify. This is not only about swapin obviously. Any caller of > __migration_entry_wait is affected the same way AFAICS. In other words. Why cannot we do the following? diff --git a/mm/migrate.c b/mm/migrate.c index f7e4bfdc13b7..7ccab29bcf9a 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -324,19 +324,9 @@ void __migration_entry_wait(struct mm_struct *mm, pte_t *ptep, goto out; page = migration_entry_to_page(entry); - - /* - * Once page cache replacement of page migration started, page_count - * *must* be zero. And, we don't want to call wait_on_page_locked() - * against a page without get_page(). - * So, we use get_page_unless_zero(), here. Even failed, page fault - * will occur again. - */ - if (!get_page_unless_zero(page)) - goto out; pte_unmap_unlock(ptep, ptl); - wait_on_page_locked(page); - put_page(page); + page_lock(page); + page_unlock(page); return; out: pte_unmap_unlock(ptep, ptl); -- Michal Hocko SUSE Labs
