Re: [PATCH] Export MDRAID bitmap on disk structure in UAPI header file

[Mariusz Tkaczyk <mtkaczyk@kernel.org>]

Sorry missclick, adding linux-raid now.
I think we don't need to spam on linux-kernel as these are MD internals.

Kuai please take a look again.

Thanks,
Mariusz

On Tue, 31 Dec 2024 12:31:08 +0100
Mariusz Tkaczyk <mtkaczyk@kernel.org> wrote:

> On Tue, 31 Dec 2024 12:00:31 +0100
> Tomáš Mudruňka <tomas.mudrunka@gmail.com> wrote:
> 
> > > > Thanks for the patch, however, Why do you want to directly
> > > > manipulate the metadata instead of using mdadm? You must first
> > > > provide an explanation to convince us that what you're doing
> > > > makes sense, and it's best to show your work.  
> > 
> > I am adding MD RAID support to genimage tool:
> > https://github.com/pengutronix/genimage/
> > 
> > It is used to generate firmware/disk images. Without such a tool it
> > is impossible to build disk image containing md raid metadata
> > without actually assembling it in the kernel via losetup or
> > something...
> > 
> > I am already using #include <linux/raid/md_p.h> which includes
> > references to bitmap structures:
> > 
> > $ grep -ri bitmap /usr/include/linux/raid/md_p.h
> > #define    MD_SB_BITMAP_PRESENT    8 /* bitmap may be present nearby
> > */ __le32    feature_map;    /* bit 0 set if 'bitmap_offset' is
> > meaningful */ __le32    bitmap_offset;    /* sectors after start of
> > superblock that bitmap starts
> >                      * NOTE: signed, so bitmap can be before
> > superblock #define MD_FEATURE_BITMAP_OFFSET    1
> > #define    MD_FEATURE_RECOVERY_BITMAP    128 /* recovery that is
> > happening
> >                          * is guided by bitmap.
> > #define    MD_FEATURE_ALL            (MD_FEATURE_BITMAP_OFFSET    \
> >                     |MD_FEATURE_RECOVERY_BITMAP    \
> > 
> > But when i use those, the resulting metadata is invalid, unless i
> > populate the structures from drivers/md/md-bitmap.h so i had to
> > copypaste its contents to my code, but i am not happy about it
> > (including half and copypasting half):
> 
> https://github.com/md-raid-utilities/mdadm/blob/main/bitmap.h
> 
> Correct me if I'm wrong but looks like it is what we did in mdadm.
> Well, If you don't want to care about it, you can consider adding
> mdadm as submodule in your application and use mdadm's headers.
> 
> Just an option, I have no hard feelings here.
> 
> Looking into that now make me more feeling that we should export this
> header long time ago instead of reimplementing it in mdadm. Kuai, what
> do you think?
> 
> > 
> > https://github.com/Harvie/genimage/blob/master/image-mdraid.c
> > 
> > > I'm with Kuai here. I would also add that for such purposes you
> > > can use externally managed metadata, not native. External
> > > management was proposed to address your problem however over the
> > > years it turned out to not be good conception (kernel driver
> > > relies on userspace daemon which is not secure).
> > >
> > > Thanks,
> > > Mariusz  
> > 
> > Hope my reply is sufficient.
> > 
> > Thank you guys!
> > Tom
> 
> Looks like old problem we get used to. If Kuai agrees too, I'm open to
> add this but.. as a mdadm maintainer (primary tool to manipulate
> mdraid) I would like you to handle this on mdadm site too to make sure
> we have it consistent and we exported exactly what is needed.
> 
> Hope, it has some sense now!
> Thanks,
> Mariusz