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Date:	Mon, 16 Sep 2013 19:00:41 +0800
From:	Zhang Yanfei <>
CC:	Tang Chen <>,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
Subject: Re: [PATCH v3 0/5] x86, memblock: Allocate memory near kernel image
 before SRAT parsed.

Hello tejun,

Could you please help reviewing the patchset? As you suggested,
we've make the patchset much simpler and cleaner.

Thanks in advance!

On 09/13/2013 05:30 PM, Tang Chen wrote:
> This patch-set is based on tj's suggestion, and not fully tested. 
> Just for review and discussion. 
> This patch-set is based on the latest kernel (3.11)
> HEAD is:
> commit d5d04bb48f0eb89c14e76779bb46212494de0bec
> Author: Linus Torvalds <>
> Date:   Wed Sep 11 19:55:12 2013 -0700
> [Problem]
> The current Linux cannot migrate pages used by the kerenl because
> of the kernel direct mapping. In Linux kernel space, va = pa + PAGE_OFFSET.
> When the pa is changed, we cannot simply update the pagetable and
> keep the va unmodified. So the kernel pages are not migratable.
> There are also some other issues will cause the kernel pages not migratable.
> For example, the physical address may be cached somewhere and will be used.
> It is not to update all the caches.
> When doing memory hotplug in Linux, we first migrate all the pages in one
> memory device somewhere else, and then remove the device. But if pages are
> used by the kernel, they are not migratable. As a result, memory used by
> the kernel cannot be hot-removed.
> Modifying the kernel direct mapping mechanism is too difficult to do. And
> it may cause the kernel performance down and unstable. So we use the following
> way to do memory hotplug.
> [What we are doing]
> In Linux, memory in one numa node is divided into several zones. One of the
> zones is ZONE_MOVABLE, which the kernel won't use.
> In order to implement memory hotplug in Linux, we are going to arrange all
> hotpluggable memory in ZONE_MOVABLE so that the kernel won't use these memory.
> To do this, we need ACPI's help.
> In ACPI, SRAT(System Resource Affinity Table) contains NUMA info. The memory
> affinities in SRAT record every memory range in the system, and also, flags
> specifying if the memory range is hotpluggable.
> (Please refer to ACPI spec 5.0 5.2.16)
> With the help of SRAT, we have to do the following two things to achieve our
> goal:
> 1. When doing memory hot-add, allow the users arranging hotpluggable as
>    (This has been done by the MOVABLE_NODE functionality in Linux.)
> 2. when the system is booting, prevent bootmem allocator from allocating
>    hotpluggable memory for the kernel before the memory initialization
>    finishes.
> The problem 2 is the key problem we are going to solve. But before solving it,
> we need some preparation. Please see below.
> [Preparation]
> Bootloader has to load the kernel image into memory. And this memory must be 
> unhotpluggable. We cannot prevent this anyway. So in a memory hotplug system, 
> we can assume any node the kernel resides in is not hotpluggable.
> Before SRAT is parsed, we don't know which memory ranges are hotpluggable. But
> memblock has already started to work. In the current kernel, memblock allocates 
> the following memory before SRAT is parsed:
> setup_arch()
>  |->memblock_x86_fill()            /* memblock is ready */
>  |......
>  |->early_reserve_e820_mpc_new()   /* allocate memory under 1MB */
>  |->reserve_real_mode()            /* allocate memory under 1MB */
>  |->init_mem_mapping()             /* allocate page tables, about 2MB to map 1GB memory */
>  |->dma_contiguous_reserve()       /* specified by user, should be low */
>  |->setup_log_buf()                /* specified by user, several mega bytes */
>  |->relocate_initrd()              /* could be large, but will be freed after boot, should reorder */
>  |->acpi_initrd_override()         /* several mega bytes */
>  |->reserve_crashkernel()          /* could be large, should reorder */
>  |......
>  |->initmem_init()                 /* Parse SRAT */
> According to Tejun's advice, before SRAT is parsed, we should try our best to
> allocate memory near the kernel image. Since the whole node the kernel resides 
> in won't be hotpluggable, and for a modern server, a node may have at least 16GB
> memory, allocating several mega bytes memory around the kernel image won't cross
> to hotpluggable memory.
> [About this patch-set]
> So this patch-set does the following:
> 1. Make memblock be able to allocate memory from low address to high address.
>    1) Keep all the memblock APIs' prototype unmodified.
>    2) When the direction is bottom up, keep the start address greater than the 
>       end of kernel image.
> 2. Improve init_mem_mapping() to support allocate page tables in bottom up direction.
> 3. Introduce "movablenode" boot option to enable and disable this functionality.
> PS: Reordering of relocate_initrd() has not been done yet. acpi_initrd_override() 
>     needs to access initrd with virtual address. So relocate_initrd() must be done 
>     before acpi_initrd_override().
> Change log v2 -> v3:
> 1. According to Toshi's suggestion, move the direction checking logic into memblock.
>    And simply the code more.
> Change log v1 -> v2:
> 1. According to tj's suggestion, implemented a new function memblock_alloc_bottom_up() 
>    to allocate memory from bottom upwards, whihc can simplify the code.
> Tang Chen (5):
>   memblock: Introduce allocation direction to memblock.
>   memblock: Improve memblock to support allocation from lower address.
>   x86, acpi, crash, kdump: Do reserve_crashkernel() after SRAT is
>     parsed.
>   x86, mem-hotplug: Support initialize page tables from low to high.
>   mem-hotplug: Introduce movablenode boot option to control memblock
>     allocation direction.
>  Documentation/kernel-parameters.txt |   15 ++++
>  arch/x86/kernel/setup.c             |   44 ++++++++++++-
>  arch/x86/mm/init.c                  |  121 ++++++++++++++++++++++++++--------
>  include/linux/memblock.h            |   22 ++++++
>  include/linux/memory_hotplug.h      |    5 ++
>  mm/memblock.c                       |  120 +++++++++++++++++++++++++++++++----
>  mm/memory_hotplug.c                 |    9 +++
>  7 files changed, 293 insertions(+), 43 deletions(-)

Zhang Yanfei
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