linux-kernel - Re: [PATCH v2] brd: fix sleeping function called from invalid context in brd_insert

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <c28dd90a-3777-49fa-a662-32c61da22860@kernel.dk>
Date: Mon, 30 Jun 2025 21:00:58 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Yu Kuai <yukuai1@...weicloud.com>, hch@....de
Cc: penguin-kernel@...ove.SAKURA.ne.jp, linux-block@...r.kernel.org,
 linux-kernel@...r.kernel.org, yi.zhang@...wei.com, yangerkun@...wei.com,
 johnny.chenyi@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH v2] brd: fix sleeping function called from invalid context
 in brd_insert_page()

On 6/30/25 7:28 PM, Yu Kuai wrote:
> Hi,
> 
> ? 2025/06/30 23:28, Jens Axboe ??:
>> On 6/30/25 9:24 AM, Jens Axboe wrote:
>>> On 6/30/25 5:28 AM, Yu Kuai wrote:
>>>> From: Yu Kuai <yukuai3@...wei.com>
>>>>
>>>> __xa_cmpxchg() is called with rcu_read_lock(), and it will allocate
>>>> memory if necessary.
>>>>
>>>> Fix the problem by moving rcu_read_lock() after __xa_cmpxchg(), meanwhile,
>>>> it still should be held before xa_unlock(), prevent returned page to be
>>>> freed by concurrent discard.
>>>
>>> The rcu locking in there is a bit of a mess, imho. What _exactly_ is the
>>> rcu read side locking protecting? Is it only needed around the lookup
>>> and insert? We even hold it over the kmap and copy, which seems very
>>> heavy handed.
>>
>> Gah it's holding the page alive too. Can't we just grab a ref to the
>> page when inserting it, and drop that at free time? It would be a lot
>> better to have only the lookup be RCU protected, having the full
>> copies under it seems kind of crazy.
> 
> In this case, we must grab a ref to the page for each read/write as
> well, I choose RCU because I think it has less performance overhead than
> page ref, which is atomic. BTW, I thought copy at most one page is
> lightweight, if this is not true, I agree page ref is better.

Right, you'd need to grab a ref. I do think that is (by far) the better
solution. Yes if you microbenchmark I'm sure the current approach will
look fine, but it's a heavy section inside an rcu read lock and will
hold off the grace period.

So yeah, I do think it'd be a lot better to do proper page references on
lookup+free, and have just the lookup be behind rcu.

-- 
Jens Axboe