Re: [PATCH] i386-pc: build btrfs zstd support into separate module

[Michael Chang]

On Tue, Oct 26, 2021 at 02:55:21PM +0200, Daniel Kiper wrote:
> On Fri, Sep 10, 2021 at 05:22:22PM +0800, Michael Chang via Grub-devel wrote:
> > On Wed, Sep 08, 2021 at 09:37:52PM +0200, Daniel Kiper wrote:
> > > On Fri, Sep 03, 2021 at 09:21:39AM +0800, Michael Chang via Grub-devel 
> > > wrote:
> > > > On Thu, Sep 02, 2021 at 02:12:52PM +0200, Daniel Kiper wrote:
> > > > > On Thu, Sep 02, 2021 at 01:48:30PM +0800, Michael Chang via 
> > > > > Grub-devel wrote:
> > > > > > On Wed, Sep 01, 2021 at 06:38:22PM +0200, Daniel Kiper wrote:
> > > > > > > On Tue, Aug 31, 2021 at 03:12:28PM +0800, Michael Chang via 
> > > > > > > Grub-devel wrote:
> >
> > [snip]
> >
> > > > just that I can't resist to fix problem from our users who opted to use
> > > > "btrfs partition" which differs to "short mbr gap" with a lot more user
> > > > base and sensible usecases.
> > > >
> > > > FWIW we want to address this problem, because mbr gap is adjustable via
> > > > re-partitioning but btrfs bootloader area is not.
> > >
> > > Huh! The "btrfs partition" sounds like not resizable MBR gap! I am not
> > > happy with it just at the beginning. Anyway, could you explain your use
> > > case in more details including example commands and why core.img seems
> > > landing in the "btrfs partition". I am especially curious about the
> > > latter because I think the "btrfs partition" and things like that was
> > > designed for something else, e.g., storing grubenv data. And why this
> > > solution is i386 specific?
> >
> > Installing to btrfs partition is not something exotic to grub, it is
> > actually a supported usecase by grub for a long time. Also zfs has
> > similar design, see:
> >
> >   c7ba4f698     Support BtrFS embedding.
> >   ba102053c     Support ZFS embedding.
> >
> > To make it happen, you just have to point the btrfs device to
> > grub-install and it will setup and embed image there. That worked quite
> > nicely until the zstd support came into play.
> >
> > On current git head, the command fails with core.img is too large.
> >
> > # ./grub-install -d ./grub-core /dev/vda2
> >   Installing for i386-pc platform.
> >   ./grub-install: warning: your core.img is unusually large.  It won't fit 
> > in the embedding area.
> >   ./grub-install: error: filesystem `btrfs' doesn't support blocklists.
> >
> > With this patch, the embedding still works but warns the user zstd is
> > disabled, also instructing them to use MBR if they need the zstd feature
> > to work.
> >
> > # ./grub-install -d ./grub-core /dev/vda2
> > Installing for i386-pc platform.
> > ./grub-install: warning: btrfs zstd compression is disabled, please change 
> > install device to disk.
> > Installation finished. No error reported.
> >
> > It is very i386-pc centric, as it is often used by legacy mbr boot code
> > with which active partition is chainloaded. Some people still thinks
> > this is the right thing to do and feels more comfortable this way to
> > manage their multiple distrubution booting, compared with os-prober.
> 
> I do not like this solution because it is similar to all the patches
> trying to solve small MBR problem. Though, after chatting with Btrfs
> folks in Oracle, it seems to me there is better solution for this issue.
> I think we should make the GRUB installer smarter. If it cannot put the
> core image in first 64 KiB before primary superblock it should install
> the core image behind the primary superblock, i.e. starting from
> 64 KiB + 4 KiB but below 1 MiB. Please take a look at [1] which nicely
> explains the Btrfs bootloader support. Does it make sense for you?

Yes. The idea is brilliant and would be the sanest solution out there.
I'll try to come up with a patch and discuss here.

> 
> Daniel
> 
> [1] 
> https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs(5)#BOOTLOADER_SUPPORT

Thanks for pointing this out. I am looking for it for a long time as I
used to come across this paragraph but not being able to make it again.

Thanks,
Michael

> 
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