Re: [PATCH 2/4] fs: add support for LOOKUP_NONBLOCK

[Jens Axboe <axboe@kernel.dk>]

On 12/15/20 8:33 AM, Matthew Wilcox wrote:
> On Tue, Dec 15, 2020 at 08:29:40AM -0700, Jens Axboe wrote:
>> On 12/15/20 5:24 AM, Matthew Wilcox wrote:
>>> On Mon, Dec 14, 2020 at 12:13:22PM -0700, Jens Axboe wrote:
>>>> +++ b/fs/namei.c
>>>> @@ -686,6 +686,8 @@ static bool try_to_unlazy(struct nameidata *nd)
>>>>  	BUG_ON(!(nd->flags & LOOKUP_RCU));
>>>>  
>>>>  	nd->flags &= ~LOOKUP_RCU;
>>>> +	if (nd->flags & LOOKUP_NONBLOCK)
>>>> +		goto out1;
>>>
>>> If we try a walk in a non-blocking context, it fails, then we punt to
>>> a thread, do we want to prohibit that thread trying an RCU walk first?
>>> I can see arguments both ways -- this may only be a temporary RCU walk
>>> failure, or we may never be able to RCU walk this path.
>>
>> In my opinion, it's not worth it trying to over complicate matters by
>> handling the retry side differently. Better to just keep them the
>> same. We'd need a lookup anyway to avoid aliasing.
> 
> but by clearing LOOKUP_RCU here, aren't you making the retry handle
> things differently?  maybe i got lost.

That's already how it works, I'm just clearing LOOKUP_NONBLOCK (which
relies on LOOKUP_RCU) when we're clearing LOOKUP_RCU. I can try and
benchmark skipping LOOKUP_RCU when we do the blocking retry, but my gut
tells me it'll be noise.

-- 
Jens Axboe