From: Xu Yu xuyu@linux.alibaba.com Subject: mm, thp: use head page in __migration_entry_wait()
We notice that hung task happens in a corner 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.
Link: https://lkml.kernel.org/r/b9836c1dd522e903891760af9f0c86a2cce987eb.162314400... 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 Acked-by: Kirill A. Shutemov kirill.shutemov@linux.intel.com Acked-by: Hugh Dickins hughd@google.com Cc: Matthew Wilcox willy@infradead.org Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
mm/migrate.c | 1 + 1 file changed, 1 insertion(+)
--- a/mm/migrate.c~mm-thp-use-head-page-in-__migration_entry_wait +++ a/mm/migrate.c @@ -295,6 +295,7 @@ void __migration_entry_wait(struct mm_st goto out;
page = migration_entry_to_page(entry); + page = compound_head(page);
/* * Once page cache replacement of page migration started, page_count _