linux-kernel - Re: [patch 3/7] fs, notify: Add file handle entry into inotify_inode

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Date:	Wed, 14 Nov 2012 11:12:59 +0000
From:	Tvrtko Ursulin <tvrtko.ursulin@...lan.co.uk>
To:	Pavel Emelyanov <xemul@...allels.com>
Cc:	Cyrill Gorcunov <gorcunov@...nvz.org>,
	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Al Viro <viro@...iv.linux.org.uk>,
	Alexey Dobriyan <adobriyan@...il.com>,
	James Bottomley <jbottomley@...allels.com>,
	Matthew Helsley <matt.helsley@...il.com>,
	aneesh.kumar@...ux.vnet.ibm.com, bfields@...ldses.org
Subject: Re: [patch 3/7] fs, notify: Add file handle entry into inotify_inode_mark

On Wednesday 14 November 2012 14:56:00 Pavel Emelyanov wrote:
> >>> How much space does a typical file system need to encode a handle? Am I
> >>> right that for must it is just a few bytes? (I just glanced at the code
> >>> so I might be wrong.) In which case, could the handle buffer be
> >>> allocated
> >>> dynamically depending on the underlying filesystem? Perhaps adding a
> >>> facility to query a filesystem about its maximum handle buffer needs? Do
> >>> you think the saving would justify this extra work?
> >> 
> >> Well, the MAX_HANDLE_SZ is taken from NFSv4 and is 128 bytes which is
> >> quite
> >> big for inotify extension indeed. The good news is that this amount of
> >> bytes seem to be required for the most descriptive fhandle -- with info
> >> about parent, etc. We don't need such, we can live with shorter handle,
> >> people said that 40 bytes was enough for that.
> >> 
> >> However, your idea about determining the handle size dynamically seems
> >> promising. As far as I can see from the code we can call for encode_fh
> >> with
> >> size equals zero and filesystem would report back the amount of bytes it
> >> requires for a handle.
> >> 
> >> We can try going this route, what do you think?
> > 
> > Sounds much better since that would only add one pointer to the watch
> > structure in the normal case.
> > 
> > Also at checkpoint time it will use only a few bytes (compared to 64) for
> > the encode buffer for most filesystems. This part is probably not that
> > important but still a win.
> 
> No, the thing is -- we need to know the handle _before_ we start checkpoint.
> More exactly -- at the time the inotify_add_watch is called. So the memory
> save would be not that big.

Ah yes, I forgot about that. But the saving is quite solid as Cyrill already 
wrote.

It is still a bit unfortunate you have to have handles allocated all the time 
just because C&R is compiled in. There is no way you could ask the filesystem 
to create you one on demand. What would you need? Just the superblock and 
inode, or more?

Regards,

Tvrtko

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