linux-kernel - Re: [PATCH 0/4] dm verity: add support for error correction

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Message-ID: <20151109191925.GA29185@google.com>
Date:	Mon, 9 Nov 2015 19:19:25 +0000
From:	Sami Tolvanen <samitolvanen@...gle.com>
To:	Mike Snitzer <snitzer@...hat.com>
Cc:	Milan Broz <mbroz@...hat.com>,
	device-mapper development <dm-devel@...hat.com>,
	Mikulas Patocka <mpatocka@...hat.com>,
	Mandeep Baines <msb@...omium.org>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	linux-kernel@...r.kernel.org, Alasdair Kergon <agk@...hat.com>,
	Mark Salyzyn <salyzyn@...gle.com>
Subject: Re: [PATCH 0/4] dm verity: add support for error correction

On Mon, Nov 09, 2015 at 11:37:35AM -0500, Mike Snitzer wrote:
> I'm left wondering: can the new error correction code be made an
> optional feature that is off by default? -- so as to preserve some
> isolation of this new code from the old dm-verity behaviour.

It's optional in the sense that you must specify error correction
parameters in the table to turn it on. Otherwise, verity_dec_decode
returns -1 and dm-verity handles errors as before.

> might be good to add a wrapper like verity_fec_is_enabled().

Sure. I can do this in v2 and address the other feedback and build
issues as well.

> Also, the 2 other big questions from Mikulas need answering:
> 1) why aren't you actually adjustng error codes, returning success, if
>    dm-verity was able to trap/correct the corruption?

We don't see actual I/O errors very often. Most corruption we've seen
is caused by flaky hardware that doesn't return errors. However, I can
certainly change to code to attempt recovery in this case too.

> 2) please fix the code to preallocate all required memory -- so that
>    verity_fec_alloc_buffers() isn't called in map.

I tried to avoid preallocating the buffers because they are relatively
large (up to 1 MiB depending on the Reed-Solomon parameters) and not
required unless we have errors to correct. I suppose there's no way to
safely do this in the middle of I/O?

> If this error correction feature is going to go upstream we really
> should see any associated userspace enablement also included in
> veritysetup.

I can look into this.

Sami
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