Re: bcache super block corruption with non 4k pages

From: Kent Overstreet
Date: Tue Jul 26 2016 - 06:22:10 EST

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.
> >
> >
> >
> > 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:

Probably best to just do something small and localized.