Re: [PATCH v2 1/6] fs: split off vfs_getdents function of getdents64 syscall

From: Dominique Martinet <asmadeus@codewreck.org>
To: Christian Brauner <brauner@kernel.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
	Jens Axboe <axboe@kernel.dk>,
	Pavel Begunkov <asml.silence@gmail.com>,
	Stefan Roesch <shr@fb.com>, Clay Harris <bugs@claycon.org>,
	Dave Chinner <david@fromorbit.com>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	io-uring@vger.kernel.org
Subject: Re: [PATCH v2 1/6] fs: split off vfs_getdents function of getdents64 syscall
Date: Thu, 25 May 2023 20:00:02 +0900	[thread overview]
Message-ID: <ZG8_su9Pq1oI-t5s@codewreck.org> (raw)
In-Reply-To: <20230525-funkanstalt-ertasten-a43443d045c8@brauner>

Christian Brauner wrote on Thu, May 25, 2023 at 11:22:08AM +0200:
> > What was confusing is that default_llseek updates f_pos under the
> > inode_lock (write), and getdents also takes that lock (for read only in
> > shared implem), so I assumed getdents also was just protected by this
> > read lock, but I guess that was a bad assumption (as I kept pointing
> > out, a shared read lock isn't good enough, we definitely agree there)
> > 
> > 
> > In practice, in the non-registered file case io_uring is also calling
> > fdget, so the lock is held exactly the same as the syscall and I wasn't
> 
> No, it really isn't. fdget() doesn't take f_pos_lock at all:
> 
> fdget()
> -> __fdget()
>    -> __fget_light()
>       -> __fget()
>          -> __fget_files()
>             -> __fget_files_rcu()

Ugh, I managed to not notice that I was looking at fdget_pos and that
it's not the same as fdget by the time I wrote two paragraphs... These
functions all have too many wrappers and too similar names for a quick
look before work.

> If that were true then any system call that passes an fd and uses
> fdget() would try to acquire a mutex on f_pos_lock. We'd be serializing
> every *at based system call on f_pos_lock whenever we have multiple fds
> referring to the same file trying to operate on it concurrently.
> 
> We do have fdget_pos() and fdput_pos() as a special purpose fdget() for
> a select group of system calls that require this synchronization.

Right, that makes sense, and invalidates everything I said after that
anyway but it's not like looking stupid ever killed anyone.

Ok so it would require adding a new wrapper from struct file to struct
fd that'd eventually take the lock and set FDPUT_POS_UNLOCK for... not
fdput_pos but another function for that stopping short of fdput...
Then just call that around both vfs_llseek and vfs_getdents calls; which
is the easy part.

(Or possibly call mutex_lock directly like Dylan did in [1]...)
[1] https://lore.kernel.org/all/20220222105504.3331010-1-dylany@fb.com/T/#m3609dc8057d0bc8e41ceab643e4d630f7b91bde6

I'll be honest though I'm thankful for your explanations but I think
I'll just do like Stefan and stop trying for now: the only reason I've
started this was because I wanted to play with io_uring for a new toy
project and it felt awkward without a getdents for crawling a tree; and
I'm long past the point where I should have thrown the towel and just
make that a sequential walk.
There's too many "conditional patches" (NOWAIT, end of dir indicator)
that I don't care about and require additional work to rebase
continuously so I'll just leave it up to someone else who does care.

So to that someone: feel free to continue from these branches (I've
included the fix for kernfs_fop_readdir that Dan Carpenter reported):
https://github.com/martinetd/linux/commits/io_uring_getdents
https://github.com/martinetd/liburing/commits/getdents

Or just start over, there's not that much code now hopefully the
baseline requirements have gotten a little bit clearer.

Sorry for stirring the mess and leaving halfway, if nobody does continue
I might send a v3 when I have more time/energy in a few months, but it
won't be quick.

-- 
Dominique