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Subject: Re: [PATCH v2 0/5] Allocate memmap from hotadded memory
From:   Rashmica Gupta <rashmica.g@gmail.com>
To:     David Hildenbrand <david@redhat.com>,
        Oscar Salvador <osalvador@suse.de>
Cc:     akpm@linux-foundation.org, mhocko@suse.com,
        dan.j.williams@intel.com, pasha.tatashin@soleen.com,
        Jonathan.Cameron@huawei.com, anshuman.khandual@arm.com,
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Date:   Tue, 02 Jul 2019 16:42:34 +1000
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Hi David,

Sorry for the late reply.

On Wed, 2019-06-26 at 10:28 +0200, David Hildenbrand wrote:
> On 26.06.19 10:15, Oscar Salvador wrote:
> > On Wed, Jun 26, 2019 at 10:11:06AM +0200, David Hildenbrand wrote:
> > > Back then, I already mentioned that we might have some users that
> > > remove_memory() they never added in a granularity it wasn't
> > > added. My
> > > concerns back then were never fully sorted out.
> > > 
> > > arch/powerpc/platforms/powernv/memtrace.c
> > > 
> > > - Will remove memory in memory block size chunks it never added
> > > - What if that memory resides on a DIMM added via
> > > MHP_MEMMAP_DEVICE?
> > > 
> > > Will it at least bail out? Or simply break?
> > > 
> > > IOW: I am not yet 100% convinced that MHP_MEMMAP_DEVICE is save
> > > to be
> > > introduced.
> > 
> > Uhm, I will take a closer look and see if I can clear your
> > concerns.
> > TBH, I did not try to use arch/powerpc/platforms/powernv/memtrace.c
> > yet.
> > 
> > I will get back to you once I tried it out.
> > 
> 
> BTW, I consider the code in arch/powerpc/platforms/powernv/memtrace.c
> very ugly and dangerous.

Yes it would be nice to clean this up.

> We should never allow to manually
> offline/online pages / hack into memory block states.
> 
> What I would want to see here is rather:
> 
> 1. User space offlines the blocks to be used
> 2. memtrace installs a hotplug notifier and hinders the blocks it
> wants
> to use from getting onlined.
> 3. memory is not added/removed/onlined/offlined in memtrace code.
>

I remember looking into doing it a similar way. I can't recall the
details but my issue was probably 'how does userspace indicate to
the kernel that this memory being offlined should be removed'?

I don't know the mm code nor how the notifiers work very well so I
can't quite see how the above would work. I'm assuming memtrace would
register a hotplug notifier and when memory is offlined from userspace,
the callback func in memtrace would be called if the priority was high
enough? But how do we know that the memory being offlined is intended
for usto touch? Is there a way to offline memory from userspace not
using sysfs or have I missed something in the sysfs interface?

On a second read, perhaps you are assuming that memtrace is used after
adding new memory at runtime? If so, that is not the case. If not, then
would you be able to clarify what I'm not seeing?

Thanks.

> CCing the DEVs.
>