Re: [PATCH 3/5] io_uring: add support for getdents

[Christian Brauner <brauner@kernel.org>]

On Mon, Jul 31, 2023 at 06:28:02PM -0600, Jens Axboe wrote:
> On 7/31/23 9:26?AM, Darrick J. Wong wrote:
> > I've watched quite a bit of NOWAIT whackamole going on over the past few
> > years (i_rwsem, the ILOCK, the IO layer, memory allocations...).  IIRC
> > these filesystem ios all have to run in process context, right?  If so,
> > why don't we capture the NOWAIT state in a PF flag?  We already do that
> > for NOFS/NOIO memory allocations to make sure that /all/ reclaim
> > attempts cannot recurse into the fs/io stacks.
> 
> I would greatly prefer passing down the context rather than capitulating
> and adding a task_struct flag for this. I think it _kind of_ makes sense
> for things like allocations, as you cannot easily track that all the way
> down, but it's a really ugly solution. It certainly creates more churn
> passing it down, but it also reveals the parts that need to check it.
> WHen new code is added, it's much more likely you'll spot the fact that
> there's passed in context. For allocation, you end up in the allocator
> anyway, which can augment the gfp mask with whatever is set in the task.
> The same is not true for locking and other bits, as they don't return a
> value to begin with. When we know they are sane, we can flag the fs as
> supporting it (like we've done for async buffered reads, for example).
> 
> It's also not an absolute thing, like memory allocations are. It's
> perfectly fine to grab a mutex under NOWAIT issue. What you should not
> do is grab a mutex that someone else can grab while waiting on IO. This
> kind of extra context is only available in the code in question, not
> generically for eg mutex locking.
> 
> I'm not a huge fan of the "let's add a bool nowait". Most/all use cases
> pass down state anyway, putting it in a suitable type struct seems much

We're only going to pass a struct if there really is a need for one
though. Meaning, we're shouldn't end up passing a struct with a single
element around in the hopes that we'll add more members at some point. 

> cleaner to me than the out-of-band approach of just adding a
> current->please_nowait.

I'm not convinced that abusing current/task_struct for this is sane. I
not just very much doubt this will go down well with reviewers outside
of fs/ we'd also rightly be told that we're punting on a design problem
because it would be more churn.