linux-kernel - Re: [PATCH 1/1] fallocate: create FAN_MODIFY and IN

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Date:	Mon, 06 Oct 2014 21:10:25 +0200
From:	Heinrich Schuchardt <xypron.glpk@....de>
To:	Jan Kara <jack@...e.cz>
CC:	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Eric Paris <eparis@...isplace.org>,
	John McCutchan <john@...nmccutchan.com>,
	Robert Love <rlove@...ve.org>,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [PATCH 1/1] fallocate: create FAN_MODIFY and IN_MODIFY events

On 06.10.2014 16:12, Jan Kara wrote:
> On Fri 03-10-14 10:19:30, Heinrich Schuchardt wrote:
>> The fanotify and the inotify API can used to monitor changes of the file
>> system.
>>
>> System call fallocate modifies files. Hence it should trigger the corresponding
>> fanotify (FAN_MODIFY) and inotify (IN_MODIFY) events.
>>
>> This patch adds the missing call to fsnotify_modify.
>    Well, there are different fallocate() commands and e.g. pure
> FALLOC_FL_KEEP_SIZE call will not change any data in the file. I'm not sure
> how much we care but I wanted to point that out...

The most interesting case is FALLOC_FL_COLLAPSE_RANGE because this value 
allows to create arbitrary file content from random data. Hence I think 
we really need to create FAN_MODIFY in this case.

As the fallocate(2) man page teaches:
After a successful call, subsequent writes into the range specified by 
offset and len are guaranteed not to fail because of lack of disk space.

So calling fallocate(fd, FALLOC_FL_KEEP_SIZE, offset, len) may result in 
different outcomes of a subsequent write depending on the values of 
offset and len.

Calling fallocate for a region already zeroed will not result in any 
data change.

I would like to compare fallocate() with write().

When we call write() we always create a FAN_MODIFY event even in the 
case of overwriting with identical data.

So event FAN_MODIFY does not provide any guarantee that data was 
actually changed.

In analogy to write() I suggest to keep the logic for fallocate() as 
trivial as possible:
If fallocate() succeeds, create IN_MODIFY and FAN_MODIFY events.

Best regards

Heinrich Schuchardt
>
>> Signed-off-by: Heinrich Schuchardt <xypron.glpk@....de>
>> ---
>>   fs/open.c | 5 +++++
>>   1 file changed, 5 insertions(+)
>>
>> diff --git a/fs/open.c b/fs/open.c
>> index d6fd3ac..03aa8e5 100644
>> --- a/fs/open.c
>> +++ b/fs/open.c
>> @@ -295,6 +295,11 @@ int do_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
>>
>>   	sb_start_write(inode->i_sb);
>>   	ret = file->f_op->fallocate(file, mode, offset, len);
>> +
>> +	/* Create inotify and fanotify events. */
>> +	if (ret == 0)
>> +		fsnotify_modify(file);
>> +
>>   	sb_end_write(inode->i_sb);
>>   	return ret;
>>   }
>> --
>> 2.1.0
>>

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