Re: [PATCH] arm64: configurable sparsemem section size

[Will Deacon]

On Thu, Apr 25, 2019 at 05:25:50PM +0200, Michal Hocko wrote:
> On Tue 23-04-19 16:38:43, Pavel Tatashin wrote:
> > sparsemem section size determines the maximum size and alignment that
> > is allowed to offline/online memory block. The bigger the size the less
> > the clutter in /sys/devices/system/memory/*. On the other hand, however,
> > there is less flexability in what granules of memory can be added and
> > removed.
> > 
> > Recently, it was enabled in Linux to hotadd persistent memory that
> > can be either real NV device, or reserved from regular System RAM
> > and has identity of devdax.
> > 
> > The problem is that because ARM64's section size is 1G, and devdax must
> > have 2M label section, the first 1G is always missed when device is
> > attached, because it is not 1G aligned.
> > 
> > Allow, better flexibility by making section size configurable.
> 
> Is there any inherent reason (64k page size?) that enforces such a large
> memsection?

I gave *vague* memories of running out of bits in the page flags if we
changed this, but that was a while back. If that's no longer the case,
then I'm open to changing the value, but I really don't want to expose
it as a Kconfig option as proposed in this patch. People won't have a
clue what to set and it doesn't help at all with the single-Image effort.

Will