linux-kernel - Re: [PATCH] update ctime and mtime for mmaped write

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Date:	Thu, 22 Feb 2007 15:50:03 -0500
From:	Peter Staubach <staubach@...hat.com>
To:	Miklos Szeredi <miklos@...redi.hu>
CC:	akpm@...ux-foundation.org, hugh@...itas.com,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] update ctime and mtime for mmaped write

Miklos Szeredi wrote:
>>> Take this example:
>>>
>>>     fd = open()
>>>     addr = mmap(.., fd)
>>>     write(fd, ...)
>>>     close(fd)
>>>     sleep(100)
>>>     msync(addr,...)
>>>     munmap(addr)
>>>
>>> The file times will be updated in write(), but with your patch, the
>>> bit in the mapping will also be set.
>>>
>>> Then in msync() the file times will be updated again, which is wrong,
>>> since the memory was _not_ modified through the mapping.
>>>       
>> This is correct.  I have updated my proposed patch to include the clearing
>> of AS_MCTIME in the routine which updates the mtime field.
>>     
>
> That doesn't really help.  Look at __generic_file_aio_write_nolock().
> file_update_time() is called before the data is written, so after the
> last write, there will be nothing to clear the flag.
>
> And even if fixed this case by moving the file_update_time() call to
> the end of the function, there's no guarantee, that some filesystem
> won't do something exotic and call set_page_dirty() indenpendently of
> write().  Good luck auditing all the set_page_dirty() calls ;)

Interesting.

No, I wouldn't want to get into worrying about set_page_dirty() calls.
Life is short and people have too much imagination...  :-)

    Thanx...

       ps

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