linux-kernel - Re: [PATCH 2/4] fsfreeze: added new file_start_write

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Date:	Fri, 26 Apr 2013 15:44:50 +0200
From:	Marco Stornelli <marco.stornelli@...il.com>
To:	Matthew Wilcox <matthew@....cx>
CC:	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Jan Harkes <jaharkes@...cmu.edu>, coda@...cmu.edu,
	linux-kernel@...r.kernel.org, linux-aio@...ck.org,
	codalist@...a.cs.cmu.edu, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH 2/4] fsfreeze: added new file_start_write_killable

Hi,

Il 26/04/2013 14:06, Matthew Wilcox ha scritto:
> On Fri, Apr 26, 2013 at 10:50:52AM +0200, Marco Stornelli wrote:
>> Replace file_start_write with file_start_write_killable where
>> possible.
>
> I feel like I'm missing context here.  Possibly because you only cc'd me
> on patch 2/4.  In particular, file_start_write doesn't exist upstream,
> so I'm not sure what it's for.  But returning 1 for non-regular files
> looks dodgy:

The patch series is based on -next due to several changes done by Al 
about fsfreeze. file_start_write_killable returns 1 because it's mainly 
a wrapper of __st_start_write. __sb_start_write returns 1 when 
everything is ok, 0 when the lock can't be gotten (we are using the 
trylock version) and _now_ a value < 0 when something happens (i.e. -EINTR).

>
>> +static inline int file_start_write_killable(struct file *file)
>> +{
>> +	if (!S_ISREG(file_inode(file)->i_mode))
>> +		return 1;
>> +	return sb_start_write_killable(file_inode(file)->i_sb);
>> +}
>
>> +++ b/fs/aio.c
>> @@ -1103,8 +1103,11 @@ static ssize_t aio_rw_vect_retry(struct kiocb *iocb, int rw, aio_rw_op *rw_op)
>>   	if (iocb->ki_pos < 0)
>>   		return -EINVAL;
>>
>> -	if (rw == WRITE)
>> -		file_start_write(file);
>> +	if (rw == WRITE) {
>> +		ret = file_start_write_killable(file);
>> +		if (ret < 0)
>> +			return ret;
>> +	}
>>   	do {
>
> So ... it's OK to do this write to pipes/directories/devices/... ?  Or is
> that check always taken care of elsewhere?  If so, why do we need this
> check?  I'm confused.  None of the callers check for the 'ret == 1' case,
> so I'm sure there's something wrong here, I just can't tell what it is.
>

See above.

>> +++ b/fs/read_write.c
>> @@ -438,17 +438,19 @@ ssize_t vfs_write(struct file *file, const char __user *buf, size_t count, loff_
>>   	ret = rw_verify_area(WRITE, file, pos, count);
>>   	if (ret >= 0) {
>>   		count = ret;
>> -		file_start_write(file);
>> -		if (file->f_op->write)
>> -			ret = file->f_op->write(file, buf, count, pos);
>> -		else
>> -			ret = do_sync_write(file, buf, count, pos);
>> +		ret = file_start_write_killable(file);
>>   		if (ret > 0) {
>> -			fsnotify_modify(file);
>> -			add_wchar(current, ret);
>> +			if (file->f_op->write)
>> +				ret = file->f_op->write(file, buf, count, pos);
>> +			else
>> +				ret = do_sync_write(file, buf, count, pos);
>> +			if (ret > 0) {
>> +				fsnotify_modify(file);
>> +				add_wchar(current, ret);
>> +			}
>> +			inc_syscw(current);
>> +			file_end_write(file);
>>   		}
>> -		inc_syscw(current);
>> -		file_end_write(file);
>>   	}
>>
>>   	return ret;
>
> I don't like it that you've increased the indentation here.  Better to do
> a preliminary patch which just converts to our normal style with gotos.  ie:
>

Ok, I can change the style here, no problem.

Marco
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