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

[Yu Kuai]

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@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.

Thanks,
Kuai

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