Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <d6546c44-04db-cbca-1523-a914670a607f@kernel.dk>
Date:   Wed, 28 Jun 2023 19:33:18 -0600
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux aarch64; rv:102.0) Gecko/20100101
 Thunderbird/102.12.0
Subject: Re: [GIT PULL] bcachefs
Content-Language: en-US
To:     Dave Chinner <david@fromorbit.com>,
        Kent Overstreet <kent.overstreet@linux.dev>
Cc:     torvalds@linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-bcachefs@vger.kernel.org,
        Christoph Hellwig <hch@lst.de>,
        Christian Brauner <brauner@kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>
References: <b02657af-5bbb-b46b-cea0-ee89f385f3c1@kernel.dk>
 <4b863e62-4406-53e4-f96a-f4d1daf098ab@kernel.dk>
 <20230628175204.oeek4nnqx7ltlqmg@moria.home.lan>
 <e1570c46-68da-22b7-5322-f34f3c2958d9@kernel.dk>
 <2e635579-37ba-ddfc-a2ab-e6c080ab4971@kernel.dk>
 <20230628221342.4j3gr3zscnsu366p@moria.home.lan>
 <d697ec27-8008-2eb6-0950-f612a602dcf5@kernel.dk>
 <20230628225514.n3xtlgmjkgapgnrd@moria.home.lan>
 <1e2134f1-f48b-1459-a38e-eac9597cd64a@kernel.dk>
 <20230628235018.ttvtzpfe42fri4yq@moria.home.lan>
 <ZJzXs6C8G2SL10vq@dread.disaster.area>
From:   Jens Axboe <axboe@kernel.dk>
In-Reply-To: <ZJzXs6C8G2SL10vq@dread.disaster.area>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 6/28/23 7:00?PM, Dave Chinner wrote:
> On Wed, Jun 28, 2023 at 07:50:18PM -0400, Kent Overstreet wrote:
>> On Wed, Jun 28, 2023 at 05:14:09PM -0600, Jens Axboe wrote:
>>> On 6/28/23 4:55?PM, Kent Overstreet wrote:
>>>>> But it's not aio (or io_uring or whatever), it's simply the fact that
>>>>> doing an fput() from an exiting task (for example) will end up being
>>>>> done async. And hence waiting for task exits is NOT enough to ensure
>>>>> that all file references have been released.
>>>>>
>>>>> Since there are a variety of other reasons why a mount may be pinned and
>>>>> fail to umount, perhaps it's worth considering that changing this
>>>>> behavior won't buy us that much. Especially since it's been around for
>>>>> more than 10 years:
>>>>
>>>> Because it seems that before io_uring the race was quite a bit harder to
>>>> hit - I only started seeing it when things started switching over to
>>>> io_uring. generic/388 used to pass reliably for me (pre backpointers),
>>>> now it doesn't.
>>>
>>> I literally just pasted a script that hits it in one second with aio. So
>>> maybe generic/388 doesn't hit it as easily, but it's surely TRIVIAL to
>>> hit with aio. As demonstrated. The io_uring is not hard to bring into
>>> parity on that front, here's one I posted earlier today for 6.5:
>>>
>>> https://lore.kernel.org/io-uring/20230628170953.952923-4-axboe@kernel.dk/
>>>
>>> Doesn't change the fact that you can easily hit this with io_uring or
>>> aio, and probably more things too (didn't look any further). Is it a
>>> realistic thing outside of funky tests? Probably not really, or at least
>>> if those guys hit it they'd probably have the work-around hack in place
>>> in their script already.
>>>
>>> But the fact is that it's been around for a decade. It's somehow a lot
>>> easier to hit with bcachefs than XFS, which may just be because the
>>> former has a bunch of workers and this may be deferring the delayed fput
>>> work more. Just hand waving.
>>
>> Not sure what you're arguing here...?
>>
>> We've had a long standing bug, it's recently become much easier to hit
>> (for multiple reasons); we seem to be in agreement on all that. All I'm
>> saying is that the existence of that bug previously is not reason to fix
>> it now.
> 
> I agree with Kent here  - the kernel bug needs to be fixed
> regardless of how long it has been around. Blaming the messenger
> (userspace, fstests, etc) and saying it should work around a
> spurious, unpredictable, undesirable and user-undebuggable kernel
> behaviour is not an acceptible solution here...

Not sure why you both are putting words in my mouth, I've merely been
arguing pros and cons and the impact of this. I even linked the io_uring
addition for ensuring that side will work better once the deferred fput
is sorted out. I didn't like the idea of fixing this through umount, and
even outlined how it could be fixed properly by ensuring we flush
per-task deferred puts on task exit.

Do I think it's a big issue? Not at all, because a) nobody has reported
it until now, and b) it's kind of a stupid case. If we can fix it with
minimal impact, should we? Yep. Particularly as the assumptions stated
in the original commit I referenced were not even valid back then.

-- 
Jens Axboe