linux-kernel - Re: [PATCH v2 0/5] Allocate memmap from hotadded memory

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Date:   Wed, 10 Jul 2019 11:14:25 +1000
From:   Rashmica Gupta <rashmica.g@...il.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Dan Williams <dan.j.williams@...el.com>,
        pasha.tatashin@...een.com, Jonathan.Cameron@...wei.com,
        anshuman.khandual@....com, Vlastimil Babka <vbabka@...e.cz>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/5] Allocate memmap from hotadded memory

Woops, looks like my phone doesn't send plain text emails :/

On Tue, Jul 2, 2019 at 6:52 PM Rashmica Gupta <rashmica.g@...il.com> wrote:
>
> On Tue, Jul 2, 2019 at 5:48 PM Oscar Salvador <osalvador@...e.de> wrote:
>>
>> 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:
>>
>
> Sure!
>
>>
>> 1) How memtrace works
>
>
>  You write the size of the chunk of memory you want into the debugfs file
> and memtrace will attempt to find a contiguous section of memory of that size
> that can be offlined. If it finds that, then the memory is removed from the
> kernel's mappings. If you want a different size, then you write that to the
> debugsfs file and memtrace will re-add the memory it first removed and then
> try to offline and remove the a chunk of the new size.
>
>
>>
>> 2) Why it was designed, what is the goal of the interface?
>> 3) When it is supposed to be used?
>>
>
> There is a hardware debugging facility (htm) on some power chips. To use
> this you need a contiguous portion of memory for the output to be dumped
> to - and we obviously don't want this memory to be simultaneously used by
> the kernel.
>
> At boot time we can portion off a section of memory for this (and not tell the
> kernel about it), but sometimes you want to be able to use the hardware
> debugging facilities and you haven't done this and you don't want to reboot
> your machine - and memtrace is the solution for this.
>
> If you're curious one tool that uses this debugging facility is here:
> https://github.com/open-power/pdbg. Relevant files are libpdbg/htm.c and src/htm.c.
>
>
>> 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.
>>
>
> Is that enough detail?
>
>>
>> Thanks
>>
>> --
>> Oscar Salvador
>> SUSE L3