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

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Date:	Tue, 26 Jul 2016 14:32:31 +0200
From:	Stefan Bader <stefan.bader@...onical.com>
To:	Kent Overstreet <kent.overstreet@...il.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, Stefan Bader <stefan.bader@...onical.com>
Subject: Re: bcache super block corruption with non 4k pages

On 26.07.2016 12:21, Kent Overstreet wrote:
> 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

I agree that this looks better and also rather large.
> 
> Probably best to just do something small and localized.
> 
So what did you think about the change I did? It seemed to be ok for 4K and 64K
at least and is rather small. And I believe that, compared to Zhengyuan's
approach this would have the benefit of not changing the superblock sector. So
it would be compatible with previously created superblocks.

-Stefan



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