From: Ivan Babrou <ivan@cloudflare.com>
To: Minchan Kim <minchan@kernel.org>
Cc: Linux MM <linux-mm@kvack.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Nitin Gupta <ngupta@vflare.org>,
Sergey Senozhatsky <senozhatsky@chromium.org>,
Jens Axboe <axboe@kernel.dk>,
linux-block@vger.kernel.org,
kernel-team <kernel-team@cloudflare.com>
Subject: Re: zram corruption due to uninitialized do_swap_page fault
Date: Wed, 16 Mar 2022 11:26:20 -0700 [thread overview]
Message-ID: <CABWYdi0jd_pG_qqAnnGK6otNNXeNoiAWtmC14Jv+tiSadJPw0w@mail.gmail.com> (raw)
In-Reply-To: <YjEOiZCLBMgbw8oc@google.com>
On Tue, Mar 15, 2022 at 3:09 PM Minchan Kim <minchan@kernel.org> wrote:
> I think the problem with CLONE_VM is following race
>
> CPU A CPU B
>
> do_swap_page do_swap_page
> SWP_SYNCHRONOUS_IO path SWP_SYNCHRONOUS_IO path
> swap_readpage original data
> swap_slot_free_notify
> delete zram entry
> swap_readpage zero data
> pte_lock
> map the *zero data* to userspace
> pte_unlock
> pte_lock
> if (!pte_same)
> goto out_nomap;
> pte_unlock
> return and next refault will
> read zero data
>
> So, CPU A and B see zero data. With patchset below, it changes
>
>
> CPU A CPU B
>
> do_swap_page do_swap_page
> SWP_SYNCHRONOUS_IO path SWP_SYNCHRONOUS_IO path
> swap_readpage original data
> pte_lock
> map the original data
> swap_free
> swap_range_free
> bd_disk->fops->swap_slot_free_notify
> swap_readpage read zero data
> pte_unlock
> pte_lock
> if (!pte_same)
> goto out_nomap;
> pte_unlock
> return and next refault will
> read correct data again
>
> Here, CPU A could read zero data from zram but that's not a bug
> (IOW, warning injected doesn't mean bug).
>
> The concern of the patch would increase memory size since it could
> increase wasted memory with compressed form in zram and uncompressed
> form in address space. However, most of cases of zram uses no
> readahead and then, do_swap_page is followed by swap_free so it will
> free the compressed from in zram quickly.
>
> Ivan, with this patch, you can see the warning you added in the zram
> but it shouldn't trigger the userspace corruption as mentioned above
> if I understand correctly.
>
> Could you test whether the patch prevent userspace broken?
I'm making an internal build and will push it to some location to see
how it behaves, but it might take a few days to get any sort of
confidence in the results (unless it breaks immediately).
I've also pushed my patch that disables SWP_SYNCHRONOUS_IO to a few
locations yesterday to see how it fares.
next prev parent reply other threads:[~2022-03-16 18:26 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-11 19:51 zram corruption due to uninitialized do_swap_page fault Ivan Babrou
2022-03-11 22:30 ` Yu Zhao
2022-03-11 22:36 ` Ivan Babrou
2022-03-14 22:40 ` Minchan Kim
2022-03-15 0:24 ` Ivan Babrou
[not found] ` <20220315043422.1846-1-hdanton@sina.com>
2022-03-15 4:54 ` Ivan Babrou
2022-03-15 4:18 ` Ivan Babrou
2022-03-15 6:57 ` Minchan Kim
2022-03-15 8:30 ` David Hildenbrand
2022-03-15 22:09 ` Minchan Kim
2022-03-16 18:26 ` Ivan Babrou [this message]
2022-03-18 16:30 ` Ivan Babrou
2022-03-18 17:32 ` Minchan Kim
2022-03-18 18:54 ` Minchan Kim
2022-03-25 2:10 ` Ivan Babrou
2022-03-25 8:33 ` David Hildenbrand
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