Messages in this thread Patch in this message | | | From | Sasha Levin <> | Subject | [PATCH 5.4 48/71] mm, thp: use head page in __migration_entry_wait() | Date | Mon, 28 Jun 2021 10:29:41 -0400 |
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From: Xu Yu <xuyu@linux.alibaba.com>
commit ffc90cbb2970ab88b66ea51dd580469eede57b67 upstream.
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@linux.alibaba.com 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> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> --- mm/migrate.c | 1 + 1 file changed, 1 insertion(+)
diff --git a/mm/migrate.c b/mm/migrate.c index 00bbe57c1ce2..5092ef2aa8a1 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -321,6 +321,7 @@ 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 -- 2.30.2
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