Re: [PATCH 2/2] drbd: increase maximum CPU mask size

["Christoph Böhmwalder" <christoph.boehmwalder@linbit.com>]

Am 02.08.26 um 13:05 schrieb Ionut Nechita:
> The cpu-mask resource option is limited to DRBD_CPU_MASK_SIZE - 1, i.e.
> 31 characters. bitmap_parse() reads the string as comma separated chunks
> of up to eight hex digits, each describing 32 bits, so 31 characters
> describe at most 112 CPUs. A full mask cannot be expressed on anything
> larger, which by now includes most two-socket machines.
> 
> Sparse masks can still reach higher CPU numbers by padding with zero
> chunks - "8,0,0,0" selects CPU 99 - but that only helps when few bits
> are set.
> 
> Userspace has already moved on. drbd-utils carries its own copy of these
> headers, where DRBD_CPU_MASK_SIZE has been 256 since 2023 ("drbd:
> increase maximum CPU mask size" by Joel Colledge, in LINBIT's
> drbd-headers repository). A drbdsetup built against those headers is
> rejected with -ERANGE by validate_nla() as soon as the mask does not fit
> in the 31 characters this driver accepts.
> 
> Raise the limit to match userspace. 255 characters allow a full mask for
> 908 CPUs: a leading chunk of three hex digits followed by 28 chunks of
> eight.
> 
> The cpu_mask member of struct res_opts grows accordingly. Two instances
> live on the stack, in drbd_adm_resource_opts() and in
> drbd_adm_new_resource(), which is uncritical at this size.
> 
> This only relaxes an upper bound, so an old drbdsetup keeps working
> against a kernel with this change, and a new drbdsetup keeps working
> against a kernel without it as long as the mask string still fits in the
> old limit.
> 
> The new size will likely be too small again eventually. Describing the
> mask with bitmap_parselist() would avoid that, but the list and hex
> formats are ambiguous for the same input - "8" means CPU 3 in one and
> CPU 8 in the other - so it would have to be a separate attribute rather
> than a reinterpretation of this one.
> 
> Signed-off-by: Ionut Nechita <ionut.nechita@windriver.com>
Thanks, this also looks correct.
Just one nit: in the commit message, you reference
drbd_adm_resource_opts and drbd_adm_new_resource; these have been
renamed to drbd_nl_resource_opts_doit and drbd_nl_new_resource_doit,
respectively, in the recent netlink refactoring commit.

But for the code itself:

Reviewed-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>

-- 
Christoph Böhmwalder
LINBIT | Keeping the Digital World Running
DRBD HA —  Disaster Recovery — Software defined Storage