From: Xu Yu <xuyu@xxxxxxxxxxxxxxxxx>
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.1623144009.git.xuyu@xxxxxxxxxxxxxxxxx
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>
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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
_
