linux-kernel - Re: [announce] vfs-scale git tree update

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Date:	Wed, 12 Jan 2011 15:06:01 +1100
From:	Nick Piggin <npiggin@...il.com>
To:	Ian Kent <raven@...maw.net>
Cc:	aelder@....com, Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <npiggin@...nel.dk>,
	Al Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [announce] vfs-scale git tree update

On Wed, Jan 12, 2011 at 2:59 PM, Ian Kent <raven@...maw.net> wrote:
> On Tue, 2011-01-11 at 11:57 -0600, Alex Elder wrote:
>> On Tue, 2011-01-11 at 08:51 -0800, Linus Torvalds wrote:
>> > On Tue, Jan 11, 2011 at 8:34 AM, Alex Elder <aelder@....com> wrote:
>> > >
>> > > FYI, when using this code, as merged by Linus, I hit the
>> > > BUG_ON() at the beginning of d_set_d_op() when it's called
>> > > by autofs4_dir_mkdir().  I managed to work around it by
>> > > just commenting out those BUG_ON() calls but it's something
>> > > that ought to get addressed properly.
>> >
>> > Yeah, removing the BUG_ON() isn't the right thing to do - it means
>> > that autofs4 is obviously setting the dentry ops twice for the same
>> > dentry.
>> >
>> > Possibly the thing could be relaxed to allow setting the _same_ d_op
>> > pointer, ie do something like
>> >
>> >    if (dentry->d_op == op)
>> >       return;
>> >
>> > at the top of that function. But looking at it, I don't think that
>> > fixes the autofs4 issue.
>>
>> That's easy enough, but it seems everybody else ensures
>> this gets done just once per dentry, and it would be nice
>> to preserve that "tightness" if possible.
>>
>> > The fact that autofs4 does "d_add()" before it sets the d_ops (or
>> > other dentry state, for that matter) looks a bit scary. To me that
>> > smells like it might get a  dentry lookup hit before it's actually
>> > fully done.
>>
>> Agreed.
>
> Isn't the parent i_mutex held during mkdir()?

Yes but there are concurrency cases allowed without i_mutex.

Lookup, for example, which ends up touching d_revalidate and
when dropping the dentry, possibly d_delete.

There seems no benefit to allowing switch of d_ops on a live
dentry, and many downsides. So the rule should just be that it
is not allowed.


> Still the order can be changed, of course.
>
>>
>> > Does it make any difference if you move the various d_add() calls down
>> > to the end of the functions to when the "dentry" has really been
>> > instantiated?
>>
>> Looking at it quickly, I don't think that would matter for
>> the case at hand.  I.e., that might be safer but it doesn't
>> address the fact that these fields are getting initialized
>> multiple times.
>
> Yeah, a hangover from changes done over time.
> Not setting the dentry op in ->lookup() should fix this.

How about negative dentries? They should be set up with d_ops
upon allocation, preferably if your operations can handle negative
dentries.

Thanks,
Nick
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