Re: [Bug 199931] New: systemd/rtorrent file data corruption when using echo 3 >/proc/sys/vm/drop_caches

Liu Bo
2018-06-06T13:38:27Z

On Wed, Jun 6, 2018 at 8:18 AM, Chris Mason <clm@fb.com> wrote:
>
>
> On 5 Jun 2018, at 16:03, Andrew Morton wrote:
>
>> (switched to email. Please respond via emailed reply-to-all, not via the
>> bugzilla web interface).
>>
>> On Tue, 05 Jun 2018 18:01:36 +0000 bugzilla-daemon@bugzilla.kernel.org
>> wrote:
>>
>>> https://bugzilla.kernel.org/show_bug.cgi?id=199931
>>>
>>> Bug ID: 199931
>>> Summary: systemd/rtorrent file data corruption when using echo
>>> 3 >/proc/sys/vm/drop_caches
>>
>>
>> A long tale of woe here. Chris, do you think the pagecache corruption
>> is a general thing, or is it possible that btrfs is contributing?
>>
>> Also, that 4.4 oom-killer regression sounds very serious.
>
>
> This week I found a bug in btrfs file write with how we handle stable pages.
> Basically it works like this:
>
> write(fd, some bytes less than a page)
> write(fd, some bytes into the same page)
> btrfs prefaults the userland page
> lock_and_cleanup_extent_if_need() <- stable pages
> wait for writeback()
> clear_page_dirty_for_io()
>
> At this point we have a page that was dirty and is now clean. That's
> normally fine, unless our prefaulted page isn't in ram anymore.
>
> iov_iter_copy_from_user_atomic() <--- uh oh
>
> If the copy_from_user fails, we drop all our locks and retry. But along the
> way, we completely lost the dirty bit on the page. If the page is dropped
> by drop_caches, the writes are lost. We'll just read back the stale
> contents of that page during the retry loop. This won't result in crc
> errors because the bytes we lost were never crc'd.
>

So we're going to carefully redirty the page under the page lock, right?

> It could result in zeros in the file because we're basically reading a hole,
> and those zeros could move around in the page depending on which part of the
> page was dirty when the writes were lost.
>

I got a question, while re-reading this page, wouldn't it read
old/stale on-disk data?

thanks,
liubo

> I spent a morning trying to trigger this with drop_caches and couldn't make
> it happen, even with schedule_timeout()s inserted and other tricks. But I
> was able to get corruptions if I manually invalidated pages in the critical
> section.
>
> I'm working on a patch, and I'll check and see if any of the other recent
> fixes Dave integrated may have a less exotic explanation.
>
> -chris
>
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