linux-kernel - Re: bcache super block corruption with non 4k pages

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Message-ID: <20160726102148.GA20130@kmo-pixel>
Date:	Tue, 26 Jul 2016 02:21:48 -0800
From:	Kent Overstreet <kent.overstreet@...il.com>
To:	Stefan Bader <stefan.bader@...onical.com>
Cc:	linux-bcache@...r.kernel.org, dm-devel@...hat.com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	liuzhengyuang521@...il.com, bcache@...ux.ewheeler.net,
	apw@...onical.com
Subject: Re: bcache super block corruption with non 4k pages

On Tue, Jul 26, 2016 at 11:51:25AM +0200, Stefan Bader wrote:
> On 21.07.2016 10:58, Stefan Bader wrote:
> > I was pointed at the thread which seems to address the same after
> > I wrote most of below text. Did not want to re-write this so please
> > bear with the odd layout.
> > 
> > https://www.redhat.com/archives/dm-devel/2016-June/msg00015.html
> > 
> > Zhengyuan tries to fix the problem by relocating the superblock on
> > disk. But I am not sure whether there is really any guarantee about
> > how __bread fills data into the buffer_head. What if there is the next
> > odd arch with 128K pages?
> > 
> > So below is an attempt to be more generic. Still I don't feel completely
> > happy with the way that a page moves (or is shared) between buffer_head
> > and biovec. What I tried to outline below is to let the register functions
> > allocate bio+biovec memory and use the in-memory sb_cache data to initialize
> > the biovec buffer.
> 
> Any opinions here? Also adding LKML as I don't seem to get through moderation on
> dm-devel.

The correct solution is to rip out the __bread() and just read the superblock by
issuing a bio, the same way all the other IO in bcache is done.

This is the way it's done in the bcache-dev branch - unfortunately, the patch
that does that in bcache-dev is big and invasive and probably not worth the
hassle to backport:

https://evilpiepirate.org/git/linux-bcache.git/commit/?h=bcache-dev&id=303eb67bffad57b4d9e71523e7df04bf258e66d1

Probably best to just do something small and localized.