Date:	Mon, 29 Oct 2012 19:55:29 +0100
From:	Jan Kara <jack@...e.cz>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Dave Chinner <david@...morbit.com>,
	"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 Mon 29-10-12 14:24:56, Ted Tso wrote:
> On Mon, Oct 29, 2012 at 03:37:05AM -0700, Andi Kleen wrote:
> > > Agreed, if we're going to add an xattr, then we might as well store
> > 
> > I don't think an xattr makes sense for this. It's sufficient to keep
> > this state in memory.
> > 
> > In general these error paths are hard to test and it's important
> > to keep them as simple as possible. Doing IO and other complexities
> > just doesn't make sense. Just have the simplest possible path
> > that can do the job.
> 
> It's actually pretty easy to test this particular one, and certainly
> one of the things I'd strongly encourage in this patch series is the
> introduction of an interface via madvise and fadvise that allows us to
> simulate an ECC hard error event.  So I don't think "it's hard to
> test" is a reason not to do the right thing.  Let's make it easy to
> test, and the include it in xfstests.  All of the core file systems
> are regularly running regression tests via xfstests, so if we define a
> standard way of testing this function (this is why I suggested
> fadvise/madvise instead of an ioctl, but in a pinch we could do this
> via an ioctl instead).
  Well, we have an error-injection framework which would be a better fit
for this functionality rather than some madvise / fadvise hack IMHO. And
test in xfstests can just check whether the error-injection code is
compiled in before running the test...

> Note that the problem that we're dealing with is buffered writes; so
> it's quite possible that the process which wrote the file, thus
> dirtying the page cache, has already exited; so there's no way we can
> guarantee we can inform the process which wrote the file via a signal
> or a error code return.  It's also possible that part of the file has
> been written out to the disk, so forcibly crashing the system and
> rebooting isn't necessarily going to save the file from being
> corrupted.
> 
> Also, if you're going to keep this state in memory, what happens if
> the inode gets pushed out of memory?  Do we pin the inode?  Or do we
> just say that it's stored into memory until some non-deterministic
> time (as far as userspace programs are concerned, but if they are
> running in a tight memory cgroup, it might be very short time later)
> suddenly the state gets lost?
> 
> I think it's fair that if there are file systems that don't have a
> single bit they can allocate in the inode, we can either accept
> Jun'ichi's suggest to just forcibly crash the system, or we can allow
> the state to be stored in memory.  But I suspect the core file systems
> that might be used by enterprise-class workloads will want to provide
> something better.
> 
> I'm not that convinced that we need to insert an xattr; after all, not
> all file systems support xattrs at all, and I think a single bit
> indicating that the file has corrupted data is sufficient.  But I
> think it would be useful to at least optionally support a persistent
> storage of this bit.
  Here I'd just note that the situation isn't that much different from a
plain old EIO we can hit during writeback. There we use in memory flag
(AS_EIO / PageError) and hope that an application which cares enough about
its data will call fsync() and thus get the error back. Sure it's not
ideal but so far nobody seemed to be bothered enough to improve on that. So
is it really fair to bother authors of this patch with it?

And regarding xattr - I think it's an overkill. Just recording the error in
the kernel log with enough detail and flagging the file (or even just the
filesystem) would be OK IMHO. Rest could be handled in userspace if someone
cares enough.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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