Re: [PATCH] 9p: Don't revert the I/O iterator after reading

[Antony Antony <antony.antony@secunet.com>]

Hi David,

On Mon, Oct 21, 2024 at 21:38:23 +0100, David Howells wrote:
> Hi Antony,
> 
> I think I may have a fix already lurking on my netfs-writeback branch for the
> next merge window.  Can you try the attached?

yes. The fix works. I rebooted a few times and no crash.

Tested-by: Antony Antony <antony.antony@secunet.com>

I am running test script in a loop over night. 

thanks,
-antony

> 
> David
> ---
> Don't revert the I/O iterator before returning from p9_client_read_once().
> netfslib doesn't require the reversion and nor doed 9P directory reading.
> 
> Make p9_client_read() use a temporary iterator to call down into
> p9_client_read_once(), and advance that by the amount read.
> 
> Reported-by: Antony Antony <antony@phenome.org>
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Eric Van Hensbergen <ericvh@kernel.org>
> cc: Latchesar Ionkov <lucho@ionkov.net>
> cc: Dominique Martinet <asmadeus@codewreck.org>
> cc: Christian Schoenebeck <linux_oss@crudebyte.com>
> cc: v9fs@lists.linux.dev
> cc: netfs@lists.linux.dev
> cc: linux-fsdevel@vger.kernel.org


> ---
>  net/9p/client.c |   10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
> 
> diff --git a/net/9p/client.c b/net/9p/client.c
> index 5cd94721d974..be59b0a94eaf 100644
> --- a/net/9p/client.c
> +++ b/net/9p/client.c
> @@ -1519,13 +1519,15 @@ p9_client_read(struct p9_fid *fid, u64 offset, struct iov_iter *to, int *err)
>  	*err = 0;
>  
>  	while (iov_iter_count(to)) {
> +		struct iov_iter tmp = *to;
>  		int count;
>  
> -		count = p9_client_read_once(fid, offset, to, err);
> +		count = p9_client_read_once(fid, offset, &tmp, err);
>  		if (!count || *err)
>  			break;
>  		offset += count;
>  		total += count;
> +		iov_iter_advance(to, count);
>  	}
>  	return total;
>  }
> @@ -1567,16 +1569,12 @@ p9_client_read_once(struct p9_fid *fid, u64 offset, struct iov_iter *to,
>  	}
>  	if (IS_ERR(req)) {
>  		*err = PTR_ERR(req);
> -		if (!non_zc)
> -			iov_iter_revert(to, count - iov_iter_count(to));
>  		return 0;
>  	}
>  
>  	*err = p9pdu_readf(&req->rc, clnt->proto_version,
>  			   "D", &received, &dataptr);
>  	if (*err) {
> -		if (!non_zc)
> -			iov_iter_revert(to, count - iov_iter_count(to));
>  		trace_9p_protocol_dump(clnt, &req->rc);
>  		p9_req_put(clnt, req);
>  		return 0;
> @@ -1596,8 +1594,6 @@ p9_client_read_once(struct p9_fid *fid, u64 offset, struct iov_iter *to,
>  			p9_req_put(clnt, req);
>  			return n;
>  		}
> -	} else {
> -		iov_iter_revert(to, count - received - iov_iter_count(to));
>  	}
>  	p9_req_put(clnt, req);
>  	return received;
>