linux-kernel - Re: [PATCH 2/3] ext4: introduce ext4_error_remove

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Message-ID: <20121029011632.GN29378@dastard>
Date:	Mon, 29 Oct 2012 12:16:32 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Theodore Ts'o <tytso@....edu>, "Luck, Tony" <tony.luck@...el.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	"Kleen, Andi" <andi.kleen@...el.com>,
	"Wu, Fengguang" <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>,
	Jun'ichi Nomura <j-nomura@...jp.nec.com>,
	Akira Fujita <a-fujita@...jp.nec.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 2/3] ext4: introduce ext4_error_remove_page

On Sat, Oct 27, 2012 at 06:16:26PM -0400, Theodore Ts'o wrote:
> On Fri, Oct 26, 2012 at 10:24:23PM +0000, Luck, Tony wrote:
> > > Well, we could set a new attribute bit on the file which indicates
> > > that the file has been corrupted, and this could cause any attempts to
> > > open the file to return some error until the bit has been cleared.
> > 
> > That sounds a lot better than renaming/moving the file.
> 
> What I would recommend is adding a 
> 
> #define FS_CORRUPTED_FL		0x01000000 /* File is corrupted */
> 
> ... and which could be accessed and cleared via the lsattr and chattr
> programs.

Except that there are filesystems that cannot implement such flags,
or require on-disk format changes to add more of those flags. This
is most definitely not a filesystem specific behaviour, so any sort
of VFS level per-file state needs to be kept in xattrs, not special
flags. Filesystems are welcome to optimise the storage of such
special xattrs (e.g. down to a single boolean flag in an inode), but
using a flag for something that dould, in fact, storage the exactly
offset and length of the corruption is far better than just storing
a "something is corrupted in this file" bit....

> > > Application programs could also get very confused when any attempt to
> > > open or read from a file suddenly returned some new error code (EIO,
> > > or should we designate a new errno code for this purpose, so there is
> > > a better indication of what the heck was going on?)
> > 
> > EIO sounds wrong ... but it is perhaps the best of the existing codes. Adding
> > a new one is also challenging too.
> 
> I think we really need a different error code from EIO; it's already
> horribly overloaded already, and if this is new behavior when the
> customers get confused and call up the distribution help desk, they
> won't thank us if we further overload EIO.  This is abusing one of the
> System V stream errno's, but no one else is using it:
> 
> #define EADV		 68  /* Advertise error */
> 
> I note that we've already added a new error code:
> 
> #define EHWPOISON 133	  /* Memory page has hardware error */
> 
> ... although the glibc shipping with Debian testing hasn't been taught
> what it is, so strerror(EHWPOISON) returns "Unknown error 133".  We
> could simply allow open(2) and stat(2) return this error, although I
> wonder if we're just better off defining a new error code.

If we are going to add special new "file corrupted" errors, we
should add EFSCORRUPTED (i.e. "filesystem corrupted") at the same
time....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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