Re: [RFC PATCH] Introduce persistent memory pool
From: Alexander Graf
Date: Mon Aug 28 2023 - 16:51:17 EST
+kexec, iommu, kvm
On 23.08.23 04:45, Stanislav Kinsburskii wrote:
+akpm, +linux-mm
On Fri, Aug 25, 2023 at 01:32:40PM +0000, Gowans, James wrote:
On Fri, 2023-08-25 at 10:05 +0200, Greg Kroah-Hartman wrote:
Thanks for adding me to this thread Greg!
On Tue, Aug 22, 2023 at 11:34:34AM -0700, Stanislav Kinsburskii wrote:
This patch addresses the need for a memory allocator dedicated to
persistent memory within the kernel. This allocator will preserve
kernel-specific states like DMA passthrough device states, IOMMU state, and
more across kexec.
The proposed solution offers a foundational implementation for potential
custom solutions that might follow. Though the implementation is
intentionally kept concise and straightforward to foster discussion and
feedback, it's fully functional in its current state.
Hi Stanislav, it looks like we're working on similar things. I'm looking
to develop a mechanism to support hypervisor live update for when KVM is
running VMs with PCI device passthrough. VMs with device passthrough
also necessitates passing and re-hydrating IOMMU state so that DMA can
continue during live update.
Planning on having an LPC session on this topic:
https://lpc.events/event/17/abstracts/1629/ (currently it's only a
submitted abstract so not sure if visible, hopefully it will be soon).
We are looking at implementing persistence across kexec via an in-memory
filesystem on top of reserved memory. This would have files for anything
that needs to be persisted. That includes files for IOMMU pgtables, for
guest memory or userspace-accessible memory.
It may be nice to solve all kexec persistence requirements with one
solution, but we can consider IOMMU separately. There are at least three
ways that this can be done:
a) carving out reserved memory for pgtables. This is done by your
proposal here, as well as my suggestion of a filesystem.
b) pre/post kexec hooks for drivers to serialise state and pass it
across in a structured format from old to new kernel.
c) Reconstructing IOMMU state in the new kernel by starting at the
hardware registers and walking the page tables. No state passing needed.
Have you considered option (b) and (c) here? One of the implications of
(b) and (c) are that they would need to hook into the buddy allocator
really early to be able to carve out the reconstructed page tables
before the allocator is used. Similar to how pkram [0] hooks in early to
carve out pages used for its filesystem.
Hi James,
We are indeed working on similar things, so thanks for chiming in.
I've seen pkram proposal as well as your comments there.
I think (b) will need some persistent-over-kexec memory to pass the
state across kexec as well as some key-value store persisted as well.
And the proposed persistent memory pool is aimed exactly for this
purpose.
Or do you imply some other way to pass driver's data accross kexec?
If I had to build this, I'd probably do it just like device tree passing
on ARM. It's a single, physically contiguous blob of data whose entry
point you pass to the target kernel. IIRC ACPI passing works similarly.
This would just be one more opaque data structure that then needs very
strict versioning and forward/backward compat guarantees.
I dind't consider (c) yet, thanks for for the pointer.
I have a question in this scope: how is PCI devices registers state is persisted
across kexec with the files system you are working on? I.e. how does
driver know, that the device shouldn't not be reinitialized?
The easiest way to do it initially would be kernel command line options
that hack up the drivers. But I suppose depending on the option we go
with, you can also use the respective "natural" path:
(a) A special metadata file that explains the state to the driver
(b) An entry in the structured file format that explains the state to
the target driver
(c) Compatible target drivers try to enumerate state from the target
device's register file
Potential applications include:
1. Allowing various in-kernel entities to allocate persistent pages from
a singular memory pool, eliminating the need for multiple region
reservations.
2. For in-kernel components that require the allocation address to be
available on kernel kexec, this address can be exposed to user space and
then passed via the command line.
Do you have specific examples of other state that needs to be passed
across? Trying to see whether tailoring specifically to the IOMMU case
is okay. Conceptually IOMMU state can be reconstructed starting with
hardware registers, not needing reserved memory. Other use-cases may not
have this option.
Well, basically it's IOMMU state and PCI devices to skip/avoid
initializing.
I bet there can be other misc (and unrelated things) like persistent
filesystems, block devices, etc. But I don't have a solid set of use
cases to present.
Would be great if you could think through the problem space until LPC so
we can have a solid conversation there :)
As you have no in-kernel users of this, it's not something we can even
consider at the moment for obvious reasons (neither would you want us
to.)
Can you make this part of a patch series that actually adds a user,
probably more than one, so that we can see if any of this even makes
sense?
I'm very keen to see this as well. The way that the IOMMU drivers are
enlightened to hook into your memory pool will likely be similar to how
they would hook into my proposal of an in-memory filesystem.
Do you have code available showing the IOMMU integration?
No, I don't have such a code yet.
But I was thinking that using such a allocator in the mempool allows
to hide this implementation under the hood of an existent generic
mechanism, which is then can be used to create persistent objects (file
system, for example) on top of it.
Unfortunately it's practically impossible to have a solid conversation
on generic mechanisms without actual users to see how they fit in with
the real world. That's Greg's answer to your patch set and I tend to
agree. What if (b) or (c) turn out much more viable? Then we've wasted a
lot of effort in shaping up the allocator for no good reason.
drivers/misc/Kconfig | 7 +
drivers/misc/Makefile | 1
drivers/misc/pmpool.c | 270 ++++++++++++++++++++++++++++++++++++++++++++++++
include/linux/pmpool.h | 20 ++++
4 files changed, 298 insertions(+)
create mode 100644 drivers/misc/pmpool.c
create mode 100644 include/linux/pmpool.h
misc is not for memory pools, as this is not a driver. please put this
in the properly location instead of trying to hide it from the mm
maintainers and subsystem :)
One of the reasons I thought a proper filesystem would be a better way
of exposing this functionality.
Yes, I see the point of having a file system for the goals you are
targeting. It looks like the right way forward.
What I'm not sure about is that persistent-over-kexec memory management and
the actual preservation mechanism should be an embedded part of this file
system.
What I'm trying to propose is a part of a generic mechanism to provide
state persistence across kexec, which can then be used by your file
system or something else.
What do you think about this approach?
IMHO we need to at least prototype each of the paths outlined above to
be able to create a sense for what works the best. I can see problems
with all of them - and I'm personally not convinced there will be a
one-size-fits-all solution yet.
That said, I am happy to see you pursue similar paths to what we have in
mind. It means there is a real gap in functionality in Linux kexec that
we need to overcome sooner or later.
Alex
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