linux-kernel - Re: [PATCH v5] fuse: Add support for passthrough read/write

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Date:	Wed, 03 Feb 2016 11:05:32 -0800
From:	Nikhilesh Reddy <reddyn@...eaurora.org>
To:	Jann Horn <jann@...jh.net>
CC:	torvalds@...ux-foundation.org, Miklos Szeredi <miklos@...redi.hu>,
	fuse-devel <fuse-devel@...ts.sourceforge.net>,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	gregkh@...uxfoundation.org, linux-fsdevel@...r.kernel.org,
	viro@...iv.linux.org.uk, Richard Weinberger <richard@....at>,
	Theodore Ts'o <tytso@....edu>, jack@...e.cz,
	Antonio SJ Musumeci <trapexit@...wn.link>, sven.utcke@....de,
	Nikolaus Rath <nikolaus@...h.org>,
	Jann Horn <jannhorn@...glemail.com>,
	Mike Shal <marfey@...il.com>
Subject: Re: [PATCH v5] fuse: Add support for passthrough read/write

On 02/01/2016 11:45 AM, Jann Horn wrote:
> On Mon, Feb 01, 2016 at 11:28:51AM -0800, Nikhilesh Reddy wrote:
>> On Mon 01 Feb 2016 11:15:56 AM PST, Jann Horn wrote:
>>> On Mon, Feb 01, 2016 at 10:56:27AM -0800, Nikhilesh Reddy wrote:
>>>> diff --git a/fs/fuse/passthrough.c b/fs/fuse/passthrough.c
>>> [...]
>>>> +static ssize_t fuse_passthrough_read_write_iter(struct kiocb *iocb,
>>>> +					    struct iov_iter *iter, int do_write)
>>>> +{
>>>> +	ssize_t ret_val;
>>>> +	struct fuse_file *ff;
>>>> +	struct file *fuse_file, *passthrough_filp;
>>>> +	struct inode *fuse_inode, *passthrough_inode;
>>>> +
>>>> +	ff = iocb->ki_filp->private_data;
>>>> +	fuse_file = iocb->ki_filp;
>>>> +	passthrough_filp = ff->passthrough_filp;
>>>> +
>>>> +	/* lock passthrough file to prevent it from being released */
>>>> +	get_file(passthrough_filp);
>>>> +	iocb->ki_filp = passthrough_filp;
>>>> +	fuse_inode = fuse_file->f_path.dentry->d_inode;
>>>> +	passthrough_inode = file_inode(passthrough_filp);
>>>> +
>>>> +	if (do_write) {
>>>> +		if (!passthrough_filp->f_op->write_iter)
>>>> +			return -EIO;
>>>> +		ret_val = passthrough_filp->f_op->write_iter(iocb, iter);
>>>> +
>>>> +		if (ret_val >= 0 || ret_val == -EIOCBQUEUED) {
>>>> +			fsstack_copy_inode_size(fuse_inode, passthrough_inode);
>>>> +			fsstack_copy_attr_times(fuse_inode, passthrough_inode);
>>>> +		}
>>>> +	} else {
>>>> +		if (!passthrough_filp->f_op->read_iter)
>>>> +			return -EIO;
>>>> +		ret_val = passthrough_filp->f_op->read_iter(iocb, iter);
>>>> +		if (ret_val >= 0 || ret_val == -EIOCBQUEUED)
>>>> +			fsstack_copy_attr_atime(fuse_inode, passthrough_inode);
>>>> +	}
>>>> +
>>>> +	iocb->ki_filp = fuse_file;
>>>> +
>>>> +	/* unlock passthrough file */
>>>> +	fput(passthrough_filp);
>>>
>>> Why the get_file() and fput() in this method? This doesn't look right. There
>>> is no lock you're releasing between get_file() and fput(). What are they
>>> intended for?
>>
>> Hi
>>
>> Thanks for reviewing the code.
>>
>> The passthrough file could be released under our feet say  if the userspace
>> fuse daemon crashed or was killed  ( while we are processing the read or the
>> write) causing bad things to happen.
>> The calls here are to increase the count temporarily  and then decrease it
>> so that we dont release in the middle of a write and everything is
>> gracefully handled...
>>
>> I have a comment right before the get_file call above saying the same thing.
>> Please let me know if you have any more questions.
>
> If that is the case, why can't the passthrough file be released before the
> get_file() call, e.g. while the core processing the filesystem read request
> is entering fuse_passthrough_read_write_iter()?
>
> As far as I can tell, you can drop the get_file() and fput() calls.
> fuse_setup_passthrough() already took a reference to the file for you, that
> reference can only be dropped in fuse_passthrough_release(), and the VFS
> ensures that no release call happens while a read or write is pending.
>
I just feel uncomfortable with dropping them. I thought they could be 
released ( i/o ) takes longer than the actual execution... but if i can 
be sure of it then maybe..

-- 
Thanks
Nikhilesh Reddy

Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
a Linux Foundation Collaborative Project.