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

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Message-ID: <a2dc2566-44e1-4460-bbff-bb813f4655d9@kernel.dk>
Date: Mon, 30 Jun 2025 09:28:28 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Yu Kuai <yukuai1@...weicloud.com>, hch@....de, yukuai3@...wei.com
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
Subject: Re: [PATCH v2] brd: fix sleeping function called from invalid context
 in brd_insert_page()

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.

IOW, I think there's room for some good cleanups here.

-- 
Jens Axboe