linux-kernel - Re: [patch 0/9] extended fdinfo via procfs series, v7

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Date:	Thu, 23 Aug 2012 16:44:27 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Al Viro <viro@...iv.linux.org.uk>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	James Bottomley <jbottomley@...allels.com>,
	Matthew Helsley <matt.helsley@...il.com>,
	aneesh.kumar@...ux.vnet.ibm.com
Subject: Re: [patch 0/9] extended fdinfo via procfs series, v7

On Thu, Aug 23, 2012 at 08:23:18AM -0400, J. Bruce Fields wrote:
> On Thu, Aug 23, 2012 at 02:43:23PM +0400, Cyrill Gorcunov wrote:
> > Hi guys,
> > 
> > here is updated version of the fdinfo via procfs series,
> > the changes from previous one are the following
> > 
> >  - fhandle is carried inside inotify mark but this feature
> >    is CONFIG dependent to not bloat the kernel for users
> >    who don't need it
> 
> As Al points out, this doesn't help much: if this feature is something a
> distro will want to provide, then in practice all their users are
> eventually going to end up with it turned on.
> 

Yes, I remember what Al has said, the problem is that this data attached
to inotify mark is not just a couple of bytes but rather about 136 bytes
per mark, and encoding this fhandle will take some cycles on mark creation
as well. Thus when in a sake of c/r we simply have no other way and are
to pay some trade off cost for c/r functionality, i don't think the
regular users (and note that CONFIG_CHECKPOINT_RESTORE is off by default)
should pay same cost for nothing. That's why I made it config dependant.
Again if you still think that making it config-option is a bad idea I'll
rip this symbols off, it's not a problem.

> Could you quantify the cost somehow?
> 

About 136 bytes per inotify mark.

> I wonder if you could get away with something less than MAX_HANDLE_SIZE?
> 128 bytes is the maximum allowable by NFSv4.  In practice I don't think
> any of our filesystems need more than 40 or so right now.

Look, Bruce, I would like to follow the limits we have #define'd in kernel,
because it makes code easier to support. I can #define some limit for
inotify fhandle but what should I print in fdinfo if say there is no
space left in buffer?

	Cyrill
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