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SubjectRe: [PATCHv2 0/7] x86_64/mm: remove bottom-up allocation style by pushing forward the parsing of mem hotplug info
On Tue, Jan 15, 2019 at 7:02 AM Dave Hansen <> wrote:
> On 1/10/19 9:12 PM, Pingfan Liu wrote:
> > Background
> > When kaslr kernel can be guaranteed to sit inside unmovable node
> > after [1].
> What does this "[1]" refer to?

> Also, can you clarify your terminology here a bit. By "kaslr kernel",
> do you mean the base address?
It should be the randomization of load address. Googled, and found out
that it is "base address".

> > But if kaslr kernel is located near the end of the movable node,
> > then bottom-up allocator may create pagetable which crosses the boundary
> > between unmovable node and movable node.
> Again, I'm confused. Do you literally mean a single page table page? I
> think you mean the page tables, but it would be nice to clarify this,
> and also explicitly state which page tables these are.
It should be page table pages. The page table is built by init_mem_mapping().

> > It is a probability issue,
> > two factors include -1. how big the gap between kernel end and
> > unmovable node's end. -2. how many memory does the system own.
> > Alternative way to fix this issue is by increasing the gap by
> > boot/compressed/kaslr*.
> Oh, you mean the KASLR code in arch/x86/boot/compressed/kaslr*.[ch]?
Sorry, and yes, code in arch/x86/boot/compressed/kaslr_64.c and kaslr.c

> It took me a minute to figure out you were talking about filenames.
> > But taking the scenario of PB level memory, the pagetable will take
> > server MB even if using 1GB page, different page attr and fragment
> > will make things worse. So it is hard to decide how much should the
> > gap increase.
> I'm not following this. If we move the image around, we leave holes.
> Why do we need page table pages allocated to cover these holes?
I means in arch/x86/boot/compressed/kaslr.c, store_slot_info() {
slot_area.num = (region->size - image_size) /CONFIG_PHYSICAL_ALIGN + 1
}. Let us denote the size of page table as "X", then the formula is
changed to slot_area.num = (region->size - image_size -X)
/CONFIG_PHYSICAL_ALIGN + 1. And it is hard to decide X due to the
above factors.

> > The following figure show the defection of current bottom-up style:
> > [startA, endA][startB, "kaslr kernel verly close to" endB][startC, endC]
> "defection"?
Oh, defect.

> > If nodeA,B is unmovable, while nodeC is movable, then init_mem_mapping()
> > can generate pgtable on nodeC, which stain movable node.
> Let me see if I can summarize this:
> 1. The kernel ASLR decompression code picks a spot to place the kernel
> image in physical memory.
> 2. Some page tables are dynamically allocated near (after) this spot.
> 3. Sometimes, based on the random ASLR location, these page tables fall
> over into the "movable node" area. Being unmovable allocations, this
> is not cool.
> 4. To fix this (on 64-bit at least), we stop allocating page tables
> based on the location of the kernel image. Instead, we allocate
> using the memblock allocator itself, which knows how to avoid the
> movable node.
Yes, you get my idea exactly. Thanks for your help to summary it. Hard
for me to express it clearly in English.

> > This patch makes it certainty instead of a probablity problem. It achieves
> > this by pushing forward the parsing of mem hotplug info ahead of init_mem_mapping().
> What does memory hotplug have to do with this? I thought this was all
> about early boot.

Put the info about memory hot plugable to memblock allocator,
initmem_init()->...->acpi_numa_memory_affinity_init(), where
memblock_mark_hotplug() does it. Later when memory allocator works, in
__next_mem_range(), it will check this info by

Thanks and regards,

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