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

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Date:	Tue, 07 Oct 2014 20:24:02 +0200
From:	Heinrich Schuchardt <xypron.glpk@....de>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Jan Kara <jack@...e.cz>, 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

Hello Andrew,

the patch in
https://lkml.org/lkml/2014/10/3/56
and cited below was reviewed by Jan Kara.

Please, add it to the MM tree.

Best regards

Heinrich Schuchardt

On 07.10.2014 20:05, Jan Kara wrote:
> On Mon 06-10-14 21:10:25, Heinrich Schuchardt wrote:
>> 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.
>    OK, makes sense. You can add:
> Reviewed-by: Jan Kara <jack@...e.cz>
>
> 								Honza
>>>> 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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