Messages in this thread | | | Subject | Re: [PATCH v2] mm, thp: use head page in __migration_entry_wait | From | Yu Xu <> | Date | Tue, 8 Jun 2021 21:22:28 +0800 |
| |
On 6/8/21 8:00 PM, Kirill A. Shutemov wrote: > On Tue, Jun 08, 2021 at 05:22:39PM +0800, Xu Yu wrote: >> We notice that hung task happens in a conner but practical scenario when >> CONFIG_PREEMPT_NONE is enabled, as follows. >> >> Process 0 Process 1 Process 2..Inf >> split_huge_page_to_list >> unmap_page >> split_huge_pmd_address >> __migration_entry_wait(head) >> __migration_entry_wait(tail) >> remap_page (roll back) >> remove_migration_ptes >> rmap_walk_anon >> cond_resched >> >> Where __migration_entry_wait(tail) is occurred in kernel space, e.g., >> copy_to_user in fstat, which will immediately fault again without >> rescheduling, and thus occupy the cpu fully. >> >> When there are too many processes performing __migration_entry_wait on >> tail page, remap_page will never be done after cond_resched. >> >> This makes __migration_entry_wait operate on the compound head page, >> thus waits for remap_page to complete, whether the THP is split >> successfully or roll back. >> >> Note that put_and_wait_on_page_locked helps to drop the page reference >> acquired with get_page_unless_zero, as soon as the page is on the wait >> queue, before actually waiting. So splitting the THP is only prevented >> for a brief interval. >> >> Fixes: ba98828088ad ("thp: add option to setup migration entries during PMD split") >> Suggested-by: Hugh Dickins <hughd@google.com> >> Signed-off-by: Gang Deng <gavin.dg@linux.alibaba.com> >> Signed-off-by: Xu Yu <xuyu@linux.alibaba.com> > > Looks good to me: > > Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> > > But there's one quirk: if split succeed we effectively wait on wrong > page to be unlocked. And it may take indefinite time if split_huge_page() > was called on the head page.
Inspired by you, I look into the codes, and have a new question (nothing to do with this patch).
If we split_huge_page_to_list on *tail* page (in fact, I haven't seen that used yet),
mm/huge_memory.c:2666 checks "VM_BUG_ON_PAGE(!PageLocked(head), head);" in split_huge_page_to_list(), while
mm/huge_memory.c:2497 does "unlock_page(subpage)", where subpage can be head in this scenario, in __split_huge_page().
My confusion is 1) how the pin on the @subpage is got outside split_huge_page_to_list()? can we ever get tail page?
2) head page is locked outside split_huge_page_to_list(), but unlocked in __split_huge_page()?
> > Maybe we should consider waking up head waiter on head page, even if it is > still locked after split? > > Something like this (untested): > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 63ed6b25deaa..f79a38e21e53 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -2535,6 +2535,9 @@ static void __split_huge_page(struct page *page, struct list_head *list, > */ > put_page(subpage); > } > + > + if (page == head) > + wake_up_page_bit(page, PG_locked); > } > > int total_mapcount(struct page *page) >
-- Thanks, Yu
| |