Re: mm/memblock: export memblock_{start/end}_of_DRAM

From: Sudarshan Rajagopalan
Date: Tue Nov 03 2020 - 07:53:24 EST

On 2020-10-29 23:41, David Hildenbrand wrote:
On 29.10.20 22:29, Sudarshan Rajagopalan wrote:
Hello all,


Hi!


Hi David.. thanks for the response as always.

We have a usecase where a module driver adds certain memory blocks using
add_memory_driver_managed(), so that it can perform memory hotplug
operations on these blocks. In general, these memory blocks aren’t
something that gets physically added later, but is part of actual RAM
that system booted up with. Meaning – we set the ‘mem=’ cmdline
parameter to limit the memory and later add the remaining ones using
add_memory*() variants.

The basic idea is to have driver have ownership and manage certain
memory blocks for hotplug operations.

So, in summary, you're still abusing the memory hot(un)plug
infrastructure from your driver - just not in a severe way as before.
And I'll tell you why, so you might understand why exposing this API
is not really a good idea and why your driver wouldn't - for example -
be upstream material.

Don't get me wrong, what you are doing might be ok in your context,
but it's simply not universally applicable in our current model.

Ordinary system RAM works different than many other devices (like PCI
devices) whereby *something* senses the device and exposes it to the
system, and some available driver binds to it and owns the memory.

Memory is detected by a driver and added to the system via e.g.,
add_memory_driver_managed(). Memory devices are created and the memory
is directly handed off to the system, to be used as system RAM as soon
as memory devices are onlined. There is no driver that "binds" memory
like other devices - it's rather the core (buddy) that uses/owns that
memory immediately after device creation.


I see.. and I agree that drivers are meant to *sense* that something changed or newly added, so that driver can check if it's the one responsible or compatible for handling this entity and binds to it. So I guess what it boils down to is - a driver that uses memory hotplug _cannot_ add/remove or have ownership of memblock boot memory, but for the newly added RAM blocks later on.

I was trying to mimic the detecting and adding of extra RAM by limiting the System RAM with "mem=XGB" as though system booted with XGB of boot memory and later add the remaining blocks (force detection and adding) using add_memorY-driver_manager(). This remaining blocks are calculated by 'physical end addr of boot memory' - 'memblock_end_of_DRAM'. The "physical end addr of boot memory" i.e. the actual RAM that bootloader informs to kernel can be obtained by scanning the 'memory' DT node.


For the driver be able to know how much memory was limited and how much
actually present, we take the delta of ‘bootmem physical end address’
and ‘memblock_end_of_DRAM’. The 'bootmem physical end address' is
obtained by scanning the reg values in ‘memory’ DT node and determining
the max {addr,size}. Since our driver is getting modularized, we won’t
have access to memblock_end_of_DRAM (i.e. end address of all memory
blocks after ‘mem=’ is applied).

What you do with "mem=" is force memory detection to ignore some of
it's detected memory.


So checking if memblock_{start/end}_of_DRAM() symbols can be exported?
Also, this information can be obtained by userspace by doing ‘cat
/proc/iomem’ and greping for ‘System RAM’. So wondering if userspace can

Not correct: with "mem=", cat /proc/iomem only shows *detected* +
added system RAM, not the unmodified detection.


That's correct - I meant 'memblock_end_of_DRAM' along with "mem=" can be calculated using 'cat /proc/iomem' which shows "detected plus added" System RAM, and not the remaining undetected one which got stripped off due to "mem=XGB". Basically, 'memblock_end_of_DRAM' address with 'mem=XGB' is {end addr of boot RAM - XGB}.. which would be same as end address of "System RAM" showed in /proc/iomem.

The reasoning for this is - if userspace can have access to such info and calculate the memblock end address, why not let drivers have this info using memblock_end_of_DRAM()?

have access to such info, can we allow kernel module drivers have access
by exporting memblock_{start/end}_of_DRAM().

Or are there any other ways where a module driver can get the end
address of system memory block?

And here is our problem: You disabled *detection* of that memory by
the responsible driver (here: core). Now your driver wants to know
what would have been detected. Assume you have memory hole in that
region - it would not work by simply looking at start/end. You're
driver is not the one doing the detection.


Regarding the memory hole - the driver can inspect the 'memory' DT node that kernel gets from ABL from RAM partition table if any such holes exist or not. I agree that if such holes exists, hot adding will fail since it needs block size to be added.
The same issue will arise if a RAM slot is added and a driver senses it and it only knows the start/end of this RAM slot (though such holes generally doesn't exists in RAM slots).

This is again something specific to our target which we make sure there are no such holes in the top most memory which is stripped off by "mem=" and later added by the driver. I agree this is not universal upstream material type, but its a method that drivers can utilize.

Another issue is: when using such memory for KVM guests, there is no
mechanism that tracks ownership of that memory - imagine another
driver wanting to use that memory. This really only works in special
environments.

Yet another issue: you cannot assume that memblock data will stay
around after boot. While we do it right now for arm64, that might
change at some point. This is also one of the reasons why we don't
export any real memblock data to drivers.


When using "mem=" you have to know the exact layout of your system RAM
and communicate the right places how that layout looks like manually:
here, to your driver.


I agree the issues mentioned here with this approach are valid from upstream POV, but we aren't trying to make a generic driver for this usecase and upstream it, but rather have it tailor made for our usecase alone where we know the layout of the System RAM (max bootmemory, no holes etc) and we utilize "mem=" and memory hotplug so that driver can add and have ownership of the remaining memory for later hotplug operations.

The clean way of doing things today is to allocate RAM and use it for
guests - e.g., using hugetlb/gigantic pages. As I said, there are
other techniques coming up to deal with minimizing struct page
overhead - if that's what you're concerned with (I still don't know
why you're removing the memory from the host when giving it to the
guest).

The overhead of strut page with hugetlb is valid, but we have other usecases outside of inter-VM sharing where we rely on memory hotplugging. In general, we want a way to be able to add/remove and offline/online a memory which is part of boot. With all the tools available - "mem=", "/proc/iomem", "memory" DT node and memory hotplug framework, a driver can still be able to achieve this and these tools that are present now does allow it.

Keeping the interVM memory sharing aside, would it be okay if memblock_end_of_DRAM() be exported? Like I mentioned before, there can be a userspace service that calculates this using 'cat /proc/iomem' and have it delivered to driver via a sysfs node. So I dont see any harm in exporting this info to driver. I agree other memblock info shouldn't be exposed outside to drivers. But I see no harm for memblock_end_of_DRAM().

I will be glad to share more info about the usecase where we use this approach if that would help, and I can check and get back on how much we can share since this is a proprietary usecase for Qualcomm.


Sudarshan

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