linux-kernel - Re: [PATCH 12/32] Unionfs: documentation updates

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Message-ID: <Pine.LNX.4.64.0709030854210.4454@fbirervta.pbzchgretzou.qr>
Date:	Mon, 3 Sep 2007 08:59:02 +0200 (CEST)
From:	Jan Engelhardt <jengelh@...putergmbh.de>
To:	Josef 'Jeff' Sipek <jsipek@...sunysb.edu>
cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, hch@...radead.org,
	viro@....linux.org.uk, bharata@...ux.vnet.ibm.com,
	j.blunck@...harburg.de, Erez Zadok <ezk@...sunysb.edu>
Subject: Re: [PATCH 12/32] Unionfs: documentation updates


On Sep 2 2007 22:20, Josef 'Jeff' Sipek wrote:
>+
>+While rebuilding Unionfs's objects, we also purge any page mappings and
>+truncate inode pages (see fs/Unionfs/dentry.c:purge_inode_data).  This is to

fs/unionfs/dentry.c

>+Unionfs maintains the following important invariant regarding mtime's,
>+ctime's, and atime's: the upper inode object's times are the max() of all of

utimes, ctimes and atimes.

>+2. Lockdep (a debugging feature) isn't aware of stacking, and so it
>+   incorrectly complains about locking problems.  The problem boils down to
>+   this: Lockdep considers all objects of a certain type to be in the same
>+   class, for example, all inodes.  Lockdep doesn't like to see a lock held
>+   on two inodes within the same task, and warns that it could lead to a
>+   deadlock.  However, stackable file systems do precisely that: they lock
>+   an upper object, and then a lower object, in a strict order to avoid
>+   locking problems; in addition, Unionfs, as a fan-out file system, may
>+   have to lock several lower inodes.  We are currently looking into Lockdep
>+   to see how to make it aware of stackable file systems.  In the mean time,

meantime

>@@ -86,5 +86,12 @@ command:
> 
> # mount -t unionfs -o remount,incgen none MOUNTPOINT
> 
>+Note that the older way of incrementing the generation number using an
>+ioctl, is no longer supported in Unionfs 2.0.  Ioctls in general are not

2.1?



	Jan
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