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@xxxxxxxxxx> Signed-off-by: Gang Deng <gavin.dg@xxxxxxxxxxxxxxxxx> Signed-off-by: Xu Yu <xuyu@xxxxxxxxxxxxxxxxx>Looks good to me: Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> 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
