Date:   Tue, 2 Jul 2019 09:48:18 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     Rashmica Gupta <rashmica.g@...il.com>
Cc:     David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org,
        mhocko@...e.com, dan.j.williams@...el.com,
        pasha.tatashin@...een.com, Jonathan.Cameron@...wei.com,
        anshuman.khandual@....com, vbabka@...e.cz, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/5] Allocate memmap from hotadded memory

On Tue, Jul 02, 2019 at 04:42:34PM +1000, Rashmica Gupta wrote:
> 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?

Hi Rashmica,

let us go the easy way here.
Could you please explain:

1) How memtrace works
2) Why it was designed, what is the goal of the interface?
3) When it is supposed to be used?

I have seen a couple of reports in the past from people running memtrace
and failing to do so sometimes, and back then I could not grasp why people
was using it, or under which circumstances was nice to have.
So it would be nice to have a detailed explanation from the person who wrote
it.

Thanks

-- 
Oscar Salvador
SUSE L3

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