[PATCH v3 09/30] block: Pre-allocate zone write plugs

[Damien Le Moal]

On 3/28/24 15:03, Christoph Hellwig wrote:
> On Thu, Mar 28, 2024 at 03:02:54PM +0900, Damien Le Moal wrote:
>> But that is the problem: "checking the zone number again" means referencing the
>> plug struct again from the lookup context while the last ref drop context is
>> freeing the plug. That race can be lost by the lookup context and lead to
>> referencing freed memory. So your solution would be OK for pre-allocated plugs
>> only.
> 
> Not if it is done in the Rcu critical section.
> 
>> For kmalloc-ed() plugs, we still need the rcu grace period for free. So we
>> can only optimize for the pre-allocated plugs...
> 
> Yes, bt it can use kfree_rcu which doesn't need the rcu_head in the
> zwplug.

Unfortunately, it does. kfree_rcu() is a 2 argument macro: address and rcu head
to use... The only thing we could drop from the plug struct is the gendisk pointer.

-- 
Damien Le Moal
Western Digital Research