Re: [PATCH v2] brd: fix sleeping function called from invalid context in brd_insert_page()

[Jens Axboe]

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@xxxxxxxxxx>
>>>>
>>>> __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