Re: [PATCH v2 3/5] mm: memory_hotplug: memblock to track partially removed vmemmap mem

[Michal Hocko <mhocko@kernel.org>]

On Mon 04-12-17 12:42:31, Andrea Reale wrote:
> On Mon  4 Dec 2017, 13:32, Michal Hocko wrote:
> > On Mon 04-12-17 11:49:09, Andrea Reale wrote:
> > > On Thu 30 Nov 2017, 15:51, Michal Hocko wrote:
> > > > On Thu 23-11-17 11:14:38, Andrea Reale wrote:
> > > > > When hot-removing memory we need to free vmemmap memory.
> > > > > However, depending on the memory is being removed, it might
> > > > > not be always possible to free a full vmemmap page / huge-page
> > > > > because part of it might still be used.
> > > > > 
> > > > > Commit ae9aae9eda2d ("memory-hotplug: common APIs to support page tables
> > > > > hot-remove") introduced a workaround for x86
> > > > > hot-remove, by which partially unused areas are filled with
> > > > > the 0xFD constant. Full pages are only removed when fully
> > > > > filled by 0xFDs.
> > > > > 
> > > > > This commit introduces a MEMBLOCK_UNUSED_VMEMMAP memblock flag, with
> > > > > the goal of using it in place of 0xFDs. For now, this will be used for
> > > > > the arm64 port of memory hot remove, but the idea is to eventually use
> > > > > the same mechanism for x86 as well.
> > > > 
> > > > Why cannot you use the same approach as x86 have? Have a look at the
> > > > vmemmap_free at al.
> > > > 
> > > 
> > > This arm64 hot-remove version (including vmemmap_free) is indeed an
> > > almost 1-to-1 port of the x86 approach. 
> > > 
> > > If you look at the first version of the patchset we submitted a while 
> > > ago (https://lkml.org/lkml/2017/4/11/540), we were initially using the
> > > x86 approach of filling unsued page structs with 0xFDs. Commenting on
> > > that, Mark suggested (and, indeed, I agree with him) that relying on a
> > > magic constant for marking some portions of physical memory was quite
> > > ugly. That is why we have used memblock for the purpose in this revised
> > > patchset.
> > > 
> > > If you have a different view and any concrete suggestion on how to
> > > improve this, it is definitely very well welcome. 
> > 
> > I would really prefer if those archictectues shared the code (and
> > concept) as much as possible. It is really a PITA to wrap your head
> > around each architectures for reasons which are not inherent to that
> > specific architecture. If you find the way how x86 is implemented ugly,
> > then all right, but making arm64 special just for the matter of taste is
> > far from ideal IMHO.
> 
> The plan is indeed to use this memblock flag in x86 hot remove as well,
> in place of the 0xFDs. The change is quite straightforward and we could
> push it in a next patchset release. Our rationale was to first use it in
> the new architecture and then, once proven stable, back port it to x86.
> 
> However, I am not in principle against of pushing it right now.

So please start with a simpler (cleanup) patch for x86. It will make the
life so much easier.
-- 
Michal Hocko
SUSE Labs