Re: [PATCH v2 0/3] md superblock write alignment on 512e devices

[Chris Unkel]

Hi Xiao,

Thanks for the excellent feedback.  Since bitmap_offset appears to be
a free-form field, it wasn't apparent to me that the bitmap never
starts within 4K of the bitmap.

I don't think it's worth worrying about a logical block size that's
more than 4K here--from what I can see logical block size larger than
the usual 4K page isn't going to happen.

I do think that it makes sense to handle the case where the physical
block size is more than 4K.  I think what you propose works, but I
think in the physical block > MAX_SB_SIZE case it makes more sense to
align the superblock writes to the physical block size (as now) rather
than rejecting the create/assemble.  Mounting with the possible
performance hit seems like a better outcome for the user in that case
than refusing to assemble.
It's the same check that would have to be written to reject the
assembly in that case and so the code shouldn't really be any more
complex.

So basically what I propose is:  if the physical block size is no
larger than MAX_SB_SIZE, pad to that; otherwise pad to to
logical_block_size, that is, replace queue_logical_block_size()
with something equivalent to:

    queue_physical_block_size(...) > MAX_SB_SIZE ?
queue_logical_block_size(...) : queue_physical_block_size(...)

which is simple, safe in all cases, doesn't reject any feasible
assembly, and generates aligned sb writes on all common current
devices (512n,4kn,512e.)

What do you think?

Regards,

  --Chris

On Sun, Nov 1, 2020 at 11:43 PM Xiao Ni <xni@xxxxxxxxxx> wrote:
>
>
>
> On 10/30/2020 04:13 AM, Christopher Unkel wrote:
> > Hello,
> >
> > Thanks for the feedback on the previous patch series.
> >
> > A updated patch series with the same function as the first patch
> > (https://lkml.org/lkml/2020/10/22/1058 "md: align superblock writes to
> > physical blocks") follows.
> >
> > As suggested, it introduces a helper function, which can be used to
> > reduce some code duplication.  It handles the case in super_1_sync()
> > where the superblock is extended by the addition of new component
> > devices.
> >
> > I think it also fixes a bug where the existing code in super_1_load()
> > ought to be rejecting the array with EINVAL: if the superblock padded
> > out to the *logical* block length runs into the bitmap.  For example, if
> > the bitmap offset is 2 (bitmap 1K after superblock) and the logical
> > block size is 4K, the superblock padded out to 4K runs into the bitmap.
> > This case may be unusual (perhaps only happens if the array is created
> > on a 512n device and then raw contents are copied onto a 4kn device) but
> > I think it is possible.
> Hi Chris
> For super1.1 and super1.2 bitmap offset is 8. It's a fixed value. So it
> should
> not have the risk?
>
> But for future maybe it has this problem. If the disk logical or
> physical block size
> is larger than 4K in future, it has data corruption risk.
> >
> > With respect to the option of simply replacing
> > queue_logical_block_size() with queue_physical_block_size(), I think
> > this can result in the code rejecting devices that can be loaded, but
> In mdadm it defines the max super size of super1 is 4096
> #define MAX_SB_SIZE 4096
> /* bitmap super size is 256, but we round up to a sector for alignment */
> #define BM_SUPER_SIZE 512
> #define MAX_DEVS ((int)(MAX_SB_SIZE - sizeof(struct mdp_superblock_1)) / 2)
> #define SUPER1_SIZE     (MAX_SB_SIZE + BM_SUPER_SIZE \
>                           + sizeof(struct misc_dev_info))
>
> It should be ok to replace queue_logical_block_size with
> queue_physical_block_size?
> Now it doesn't check physical block size and super block size. For
> super1, we can add
> a check that if physical block size is larger than MAX_SB_SIZE, then we
> reject to create/assmble
> the raid device.
> > for which the physical block alignment can't be respected--the longer
> > padded size would trigger the EINVAL cases testing against
> > data_offset/new_data_offset.  I think it's better to proceed in such
> > cases, just with unaligned superblock writes as would presently happen.
> > Also if I'm right about the above bug, then I think this subsitution
> > would be more likely to trigger it.
> >
> > Thanks,
> >
> >    --Chris
> >
> >
> > Christopher Unkel (3):
> >    md: factor out repeated sb alignment logic
> >    md: align superblock writes to physical blocks
> >    md: reuse sb length-checking logic
> >
> >   drivers/md/md.c | 69 +++++++++++++++++++++++++++++++++++++------------
> >   1 file changed, 52 insertions(+), 17 deletions(-)
> >
>