[PATCH v2] mm, thp: use head page in __migration_entry_wait

[Xu Yu]

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>
---
 mm/migrate.c | 1 +
 1 file changed, 1 insertion(+)

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
-- 
2.20.1.2432.ga663e714