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

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Message-ID: <773a49cf-3908-85d2-5693-5cbd6530a933@huaweicloud.com>
Date: Tue, 1 Jul 2025 09:28:59 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Jens Axboe <axboe@...nel.dk>, 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()

Hi,

在 2025/06/30 23:28, Jens Axboe 写道:
> 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.

Thanks,
Kuai

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