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Date:   Wed, 27 Sep 2023 13:44:38 +0800
From:   Baoquan He <bhe@redhat.com>
To:     Stanislav Kinsburskii <skinsburskii@linux.microsoft.com>
Cc:     tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com,
        ebiederm@xmission.com, akpm@linux-foundation.org,
        stanislav.kinsburskii@gmail.com, corbet@lwn.net,
        linux-kernel@vger.kernel.org, kexec@lists.infradead.org,
        linux-mm@kvack.org, kys@microsoft.com, jgowans@amazon.com,
        wei.liu@kernel.org, arnd@arndb.de, gregkh@linuxfoundation.org,
        graf@amazon.de, pbonzini@redhat.com
Subject: Re: [RFC PATCH v2 0/7] Introduce persistent memory pool
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Hi Stanislav,

On 09/25/23 at 02:27pm, Stanislav Kinsburskii wrote:
> This patch introduces a memory allocator specifically tailored for
> persistent memory within the kernel. The allocator maintains
> kernel-specific states like DMA passthrough device states, IOMMU state, and
> more across kexec.

Can you give more details about how this persistent memory pool will be
utilized in a actual scenario? I mean, what problem have you met so that
you have to introduce persistent memory pool to solve it?

Thanks
Baoquan

> 
> The current implementation provides a foundation for custom solutions that
> may be developed in the future. Although the design is kept concise and
> straightforward to encourage discussion and feedback, it remains fully
> functional.
> 
> The persistent memory pool builds upon the continuous memory allocator
> (CMA) and ensures CMA state persistency across kexec by incorporating the
> CMA bitmap into the memory region instead of allocation it from kernel
> memory.
> 
> Persistent memory pool metadata is passed across kexec by using Flattened
> Device Tree, which is added as another kexec segment for x86 architecture.
> 
> Potential applications include:
> 
>   1. Enabling various in-kernel entities to allocate persistent pages from
>      a unified memory pool, obviating the need for reserving multiple
>      regions.
> 
>   2. For in-kernel components that need the allocation address to be
>      retained on kernel kexec, this address can be exposed to user space
>      and subsequently passed through the command line.
> 
>   3. Distinct subsystems or drivers can set aside their region, allocating
>      a segment for their persistent memory pool, suitable for uses such as
>      file systems, key-value stores, and other applications.
> 
> Notes:
> 
>   1. The last patch of the series represents a use case for the feature.
>      However, the patch won't compile and is for illustrative purposes only
>      as the code being patched hasn't been merged yet.
> 
>   2. The code being patched is currently under review by the community. The
>      series is named "Introduce /dev/mshv drivers":
> 
>          https://lkml.org/lkml/2023/9/22/1117
> 
> 
> Changes since v1:
> 
>   1. Persistent memory pool is now a wrapper on top of CMA instead of being a
>      new allocator.
> 
>   2. Persistent memory pool metadata doesn't belong to the pool anymore and
>      is now passed via Flattened Device Tree instead over kexec to the new
>      kernel.
> 
> The following series implements...
> 
> ---
> 
> Stanislav Kinsburskii (7):
>       kexec_file: Add fdt modification callback support
>       x86: kexec: Transfer existing fdt to the new kernel
>       x86: kexec: Enable fdt modification in callbacks
>       pmpool: Introduce persistent memory pool
>       pmpool: Update device tree on kexec
>       pmpool: Restore state from device tree post-kexec
>       Drivers: hv: Allocate persistent pages for root partition
> 
> 
>  arch/x86/Kconfig                  |   16 +++
>  arch/x86/kernel/kexec-bzimage64.c |   97 +++++++++++++++++
>  drivers/hv/hv_common.c            |   13 ++
>  include/linux/kexec.h             |    7 +
>  include/linux/pmpool.h            |   22 ++++
>  kernel/kexec_file.c               |   24 ++++
>  mm/Kconfig                        |    9 ++
>  mm/Makefile                       |    1 
>  mm/pmpool.c                       |  208 +++++++++++++++++++++++++++++++++++++
>  9 files changed, 394 insertions(+), 3 deletions(-)
>  create mode 100644 include/linux/pmpool.h
>  create mode 100644 mm/pmpool.c
> 
> 
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