linux-kernel - Re: [PATCH v5 00/10] mm: Sub-section memory hotplug support

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Date:   Mon, 25 Mar 2019 15:50:23 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Jeff Moyer <jmoyer@...hat.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Toshi Kani <toshi.kani@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        stable <stable@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 00/10] mm: Sub-section memory hotplug support

On Mon 25-03-19 10:28:00, Jeff Moyer wrote:
> Michal Hocko <mhocko@...nel.org> writes:
> 
> >> > and I would like to know that you are
> >> > not just shifting the problem to a smaller unit and a new/creative HW
> >> > will force us to go even more complicated.
> >> 
> >> HW will not do this to us. It's software that has the problem.
> >> Namespace creation is unnecessarily constrained to 128MB alignment.
> >
> > And why is that a problem? A lack of documentation that this is a
> > requirement? Something will not work with a larger alignment? Someting
> > else?
> 
> See this email for one user-visible problem:
>   https://lore.kernel.org/lkml/x49imxbx22d.fsf@segfault.boston.devel.redhat.com/

: # ndctl create-namespace -m fsdax -s 128m
:   Error: '--size=' must align to interleave-width: 6 and alignment: 2097152
:   did you intend --size=132M?
: 
: failed to create namespace: Invalid argument

So the size is in section size units. So what prevents the userspace to
use a proper alignment? I am sorry if this is a stupid question but I am
not really familiar with ndctl nor the pmem side of it.
-- 
Michal Hocko
SUSE Labs