diff options
authorXu Yu <>2021-06-15 18:23:42 -0700
committerLinus Torvalds <>2021-06-16 09:24:42 -0700
commitffc90cbb2970ab88b66ea51dd580469eede57b67 (patch)
parent1b3865d016815cbd69a1879ca1c8a8901fda1072 (diff)
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: Fixes: ba98828088ad ("thp: add option to setup migration entries during PMD split") Suggested-by: Hugh Dickins <> Signed-off-by: Gang Deng <> Signed-off-by: Xu Yu <> Acked-by: Kirill A. Shutemov <> Acked-by: Hugh Dickins <> Cc: Matthew Wilcox <> Cc: <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <>
1 files changed, 1 insertions, 0 deletions
diff --git a/mm/migrate.c b/mm/migrate.c
index b234c3f3acb7..41ff2c9896c4 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -295,6 +295,7 @@ void __migration_entry_wait(struct mm_struct *mm, pte_t *ptep,
goto out;
page = migration_entry_to_page(entry);
+ page = compound_head(page);
* Once page cache replacement of page migration started, page_count