linux-kernel - Re: A possible flaw in the fsnotify design.

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Message-ID: <AANLkTind3PsDNeEB-FXADOdmPqARSKo7QNTToeGx3LG5@mail.gmail.com>
Date:	Tue, 16 Nov 2010 02:15:23 +0300
From:	Alexey Zaytsev <alexey.zaytsev@...il.com>
To:	Eric Paris <eparis@...hat.com>
Cc:	Tvrtko Ursulin <tvrtko.ursulin@...hos.com>,
	linux-fsdevel@...r.kernel.org,
	"stefan@...ttcher.org" <stefan@...ttcher.org>,
	linux-kernel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
	agruen@...bit.com
Subject: Re: A possible flaw in the fsnotify design.

On Tue, Nov 16, 2010 at 02:12, Eric Paris <eparis@...hat.com> wrote:
> On Tue, 2010-11-16 at 02:03 +0300, Alexey Zaytsev wrote:
>> On Tue, Nov 16, 2010 at 01:51, Eric Paris <eparis@...hat.com> wrote:
>> > On Tue, 2010-11-16 at 01:44 +0300, Alexey Zaytsev wrote:
>> >> On Tue, Nov 16, 2010 at 01:11, Eric Paris <eparis@...hat.com> wrote:
>> >> > On Tue, 2010-11-16 at 01:05 +0300, Alexey Zaytsev wrote:
>> >> >> Just some thoughts.
>> >> >>
>> >> >> Consider the situation: Files A and B both point to the same inode.
>> >> >> File A is being watched, but the user won't get notifications if B is
>> >> >> modified.
>> >> >
>> >> > That's not true.  Users watch inodes, not files (this is true for both
>> >> > inotify and fanotify).  Give it a try, it works.
>> >> >
>> >>
>> >> debian-i386:~/tmp# touch a
>> >> debian-i386:~/tmp# ../fanotify a &
>> >> debian-i386:~/tmp# link a b
>> >> debian-i386:~/tmp# ls -li
>> >> total 0
>> >> 3433 -rw-r--r-- 2 root root 0 Nov 15 22:37 a
>> >> 3433 -rw-r--r-- 2 root root 0 Nov 15 22:37 b
>> >> debian-i386:~/tmp# echo 123 > b
>> >> /root/tmp/b: pid=2143 mask = 20 open
>> >> /root/tmp/b: pid=2143 mask = a modify 0 - 4 close(writable)  0 - 4
>> >>
>> >> Am I doing something wrong? Same thing happens if I watch the mount point.
>> >
>> > Maybe I don't understand the problem, you watched the inode behind A.
>> > You made changes accessing this inode via B, you got notification about
>> > those changes.  Isn't that what you wanted?
>>
>> I'd expect to get two notifications in this case. Might not be a
>> problem when you are watching individual files, but there is no clear
>> way to get all the modified files, if you are watching a mount point.
>
> Ah, you were hoping for 4 events.  Yeah, not possible.  You get notified
> when the inode changes, which way you get notified is up to the kernel
> and we leave it as an (impossible) exercise to userspace to map hard
> linked inodes back together   :)
>
Yeah, I see now, it's impossible to get all the files linking to an
inode even from the kernel space without scanning the fs. Thanks for
the clarification.
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