Subject: Re: [PATCH] net: 9p: free what was emitted when read count is 0

From: Jisheng Zhang <[email-masked]>
Date: Mon, 1 Mar 2021 11:01:57 +0800

On Mon, 1 Mar 2021 11:51:24 +0900 Dominique Martinet wrote:

> 
> 
> Jisheng Zhang wrote on Mon, Mar 01, 2021 at 10:33:36AM +0800:
> > I met below warning when cating a small size(about 80bytes) txt file
> > on 9pfs(msize=2097152 is passed to 9p mount option), the reason is we
> > miss iov_iter_advance() if the read count is 0, so we didn't truncate
> > the pipe, then iov_iter_pipe() thinks the pipe is full. Fix it by
> > calling iov_iter_advance() on the iov_iter "to" even if read count is 0  
> 
> Hm, there are plenty of other error cases that don't call
> iov_iter_advance() and shouldn't trigger this warning ; I'm not sure
> just adding one particular call to this is a good solution.

Per my understanding of iov_iter, we need to call iov_iter_advance()
even when the read out count is 0. I believe we can see this common style
in other fs.

> 
> 
> How reproducible is this? From the description it should happen

100%

> everytime you cat a small file? (I'm surprised cat uses sendfile, what

it happened every time when catting a small file.

> cat version? coreutils' doesn't seem to do that on their git)

busybox cat

> 
> What kernel version do you get this on? Bonus points if you can confirm

5.11 and the latest linus tree

> this didn't use to happen, and full points for a bisect.
> 
> 
> (cat on a small file is something I do all the time in my tests, I'd
> like to be able to reproduce to understand the issue better as I'm not
> familiar with that part of the code)

Per my check, it can be 100% reproduced with busybox cat + "msize=2097152"
mount option. NOTE: msize=2097152 isn't a magic number it can be other
numbers which can ensure zerocopy code path is executed: p9_client_read_once
->p9_client_zc_rpc()

Thanks