From: Tim Chen <tim.c.chen@linux.intel.com>
To: Miaohe Lin <linmiaohe@huawei.com>, akpm@linux-foundation.org
Cc: hannes@cmpxchg.org, mhocko@suse.com, iamjoonsoo.kim@lge.com,
vbabka@suse.cz, alex.shi@linux.alibaba.com, willy@infradead.org,
minchan@kernel.org, richard.weiyang@gmail.com,
ying.huang@intel.com, hughd@google.com,
linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 2/5] swap: fix do_swap_page() race with swapoff
Date: Thu, 8 Apr 2021 14:34:30 -0700 [thread overview]
Message-ID: <7684b3de-2824-9b1f-f033-d4bc14f9e195@linux.intel.com> (raw)
In-Reply-To: <20210408130820.48233-3-linmiaohe@huawei.com>
On 4/8/21 6:08 AM, Miaohe Lin wrote:
> When I was investigating the swap code, I found the below possible race
> window:
>
> CPU 1 CPU 2
> ----- -----
> do_swap_page
> synchronous swap_readpage
> alloc_page_vma
> swapoff
> release swap_file, bdev, or ...
Perhaps I'm missing something. The release of swap_file, bdev etc
happens after we have cleared the SWP_VALID bit in si->flags in destroy_swap_extents
if I read the swapoff code correctly.
> swap_readpage
> check sis->flags is ok
> access swap_file, bdev...[oops!]
> si->flags = 0
This happens after we clear the si->flags
synchronize_rcu()
release swap_file, bdev, in destroy_swap_extents()
So I think if we have get_swap_device/put_swap_device in do_swap_page,
it should fix the race you've pointed out here.
Then synchronize_rcu() will wait till we have completed do_swap_page and
call put_swap_device.
>
> Using current get/put_swap_device() to guard against concurrent swapoff for
> swap_readpage() looks terrible because swap_readpage() may take really long
> time. And this race may not be really pernicious because swapoff is usually
> done when system shutdown only. To reduce the performance overhead on the
> hot-path as much as possible, it appears we can use the percpu_ref to close
> this race window(as suggested by Huang, Ying).
I think it is better to break this patch into two.
One patch is to fix the race in do_swap_page and swapoff
by adding get_swap_device/put_swap_device in do_swap_page.
The second patch is to modify get_swap_device and put_swap_device
with percpu_ref. But swapoff is a relatively rare events.
I am not sure making percpu_ref change for performance is really beneficial.
Did you encounter a real use case where you see a problem with swapoff?
The delay in swapoff is primarily in try_to_unuse to bring all
the swapped off pages back into memory. Synchronizing with other
CPU for paging in probably is a small component in overall scheme
of things.
Thanks.
Tim
next prev parent reply other threads:[~2021-04-08 21:34 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-08 13:08 [PATCH 0/5] close various race windows for swap Miaohe Lin
2021-04-08 13:08 ` [PATCH 1/5] mm/swapfile: add percpu_ref support " Miaohe Lin
2021-04-12 3:30 ` Huang, Ying
2021-04-12 6:59 ` Miaohe Lin
2021-04-12 7:24 ` Huang, Ying
2021-04-13 12:39 ` Miaohe Lin
2021-04-14 1:17 ` Huang, Ying
2021-04-14 1:58 ` Miaohe Lin
2021-04-14 2:06 ` Huang, Ying
2021-04-14 3:44 ` Dennis Zhou
2021-04-14 3:59 ` Huang, Ying
2021-04-14 4:05 ` Dennis Zhou
2021-04-14 5:44 ` Huang, Ying
2021-04-14 14:53 ` Dennis Zhou
2021-04-15 3:16 ` Miaohe Lin
2021-04-15 4:20 ` Dennis Zhou
2021-04-15 9:17 ` Miaohe Lin
2021-04-15 5:24 ` Huang, Ying
2021-04-15 14:31 ` Dennis Zhou
2021-04-16 0:54 ` Huang, Ying
2021-04-16 2:27 ` Miaohe Lin
2021-04-16 6:25 ` Huang, Ying
2021-04-16 8:30 ` Miaohe Lin
2021-04-08 13:08 ` [PATCH 2/5] swap: fix do_swap_page() race with swapoff Miaohe Lin
2021-04-08 21:34 ` Tim Chen [this message]
2021-04-09 8:42 ` Miaohe Lin
2021-04-09 17:17 ` Tim Chen
2021-04-10 3:17 ` Miaohe Lin
2021-04-12 1:44 ` Huang, Ying
2021-04-12 3:24 ` Miaohe Lin
2021-04-13 1:27 ` Huang, Ying
2021-04-13 19:24 ` Tim Chen
2021-04-14 1:04 ` Huang, Ying
2021-04-14 2:20 ` Miaohe Lin
2021-04-14 16:13 ` Tim Chen
2021-04-15 3:19 ` Miaohe Lin
2021-04-14 2:55 ` Miaohe Lin
2021-04-14 3:07 ` Huang, Ying
2021-04-14 3:27 ` Miaohe Lin
2021-04-08 13:08 ` [PATCH 3/5] mm/swap_state: fix get_shadow_from_swap_cache() " Miaohe Lin
2021-04-13 1:33 ` Huang, Ying
2021-04-14 2:42 ` Miaohe Lin
2021-04-08 13:08 ` [PATCH 4/5] mm/swap_state: fix potential faulted in race in swap_ra_info() Miaohe Lin
2021-04-09 8:50 ` Huang, Ying
2021-04-09 9:00 ` Miaohe Lin
2021-04-12 0:55 ` Huang, Ying
2021-04-12 3:17 ` Miaohe Lin
2021-04-08 13:08 ` [PATCH 5/5] mm/swap_state: fix swap_cluster_readahead() race with swapoff Miaohe Lin
2021-04-13 1:36 ` Huang, Ying
2021-04-14 2:43 ` Miaohe Lin
2021-04-08 14:55 ` [PATCH 0/5] close various race windows for swap riteshh
2021-04-09 8:01 ` Miaohe Lin
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