On 02/06/2013 02:10 PM, Tang Chen wrote:

On 02/06/2013 05:17 PM, Tang Chen wrote:

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Hi all,

On 02/06/2013 11:07 AM, Tang Chen wrote:

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Hi Glauber, all,

An old thing I want to discuss with you. :)

On 01/09/2013 11:09 PM, Glauber Costa wrote:

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memory can't be offlined when CONFIG_MEMCG is selected.
For example: there is a memory device on node 1. The address range
is [1G, 1.5G). You will find 4 new directories memory8, memory9,
memory10,
and memory11 under the directory /sys/devices/system/memory/.

If CONFIG_MEMCG is selected, we will allocate memory to store page
cgroup
when we online pages. When we online memory8, the memory stored
page cgroup
is not provided by this memory device. But when we online memory9,
the memory
stored page cgroup may be provided by memory8. So we can't offline
memory8
now. We should offline the memory in the reversed order.

When the memory device is hotremoved, we will auto offline memory
provided
by this memory device. But we don't know which memory is onlined
first, so
offlining memory may fail. In such case, iterate twice to offline
the memory.
1st iterate: offline every non primary memory block.
2nd iterate: offline primary (i.e. first added) memory block.

This idea is suggested by KOSAKI Motohiro.

Signed-off-by: Wen Congyang<redacted>

Maybe there is something here that I am missing - I admit that I came
late to this one, but this really sounds like a very ugly hack, that
really has no place in here.

Retrying, of course, may make sense, if we have reasonable belief
that
we may now succeed. If this is the case, you need to document - in
the
code - while is that.

The memcg argument, however, doesn't really cut it. Why can't we make
all page_cgroup allocations local to the node they are describing? If
memcg is the culprit here, we should fix it, and not retry. If
there is
still any benefit in retrying, then we retry being very specific
about why.

We try to make all page_cgroup allocations local to the node they are
describing
now. If the memory is the first memory onlined in this node, we will
allocate
it from the other node.

For example, node1 has 4 memory blocks: 8-11, and we online it from 8
to 11
1. memory block 8, page_cgroup allocations are in the other nodes
2. memory block 9, page_cgroup allocations are in memory block 8

So we should offline memory block 9 first. But we don't know in which
order
the user online the memory block.

I think we can modify memcg like this:
allocate the memory from the memory block they are describing

I am not sure it is OK to do so.

I don't see a reason why not.

You would have to tweak a bit the lookup function for page_cgroup, but
assuming you will always have the pfns and limits, it should be easy
to do.

I think the only tricky part is that today we have a single
node_page_cgroup, and we would of course have to have one per memory
block. My assumption is that the number of memory blocks is limited and
likely not very big. So even a static array would do.

About the idea "allocate the memory from the memory block they are
describing",

online_pages()
|-->memory_notify(MEM_GOING_ONLINE, &arg) ----------- memory of this
section is not in buddy yet.
|-->page_cgroup_callback()
|-->online_page_cgroup()
|-->init_section_page_cgroup()
|-->alloc_page_cgroup() --------- allocate page_cgroup from buddy
system.

When onlining pages, we allocate page_cgroup from buddy. And the being
onlined pages are not in
buddy yet. I think we can reserve some memory in the section for
page_cgroup, and return all the
rest to the buddy.

But when the system is booting,

start_kernel()
|-->setup_arch()
|-->mm_init()
| |-->mem_init()
| |-->numa_free_all_bootmem() -------------- all the pages are in buddy
system.
|-->page_cgroup_init()
|-->init_section_page_cgroup()
|-->alloc_page_cgroup() ------------------ I don't know how to reserve
memory in each section.

So any idea about how to deal with it when the system is booting please?

How about this way.

1) Add a new flag PAGE_CGROUP_INFO, like SECTION_INFO and
MIX_SECTION_INFO.
2) In sparse_init(), reserve some beginning pages of each section as
bootmem.

Hi all,

After digging into bootmem code, I met another problem.

memblock allocates memory from high address to low address, using
memblock.current_limit
to remember where the upper limit is. What I am doing will produce a lot
of fragments,
and the memory will be non-contiguous. So we need to modify memblock again.

I don't think it's a good idea. How do you think ?

Thanks. :)

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3) In register_page_bootmem_info_section(), set these pages as
page->lru.next = PAGE_CGROUP_INFO;

Then these pages will not go to buddy system.

But I do worry about the fragment problem because part of each section
will
be used in the very beginning.

Thanks. :)

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And one more question, a memory section is 128MB in Linux. If we reserve
part of the them for page_cgroup,
then anyone who wants to allocate a contiguous memory larger than 128MB,
it will fail, right ?
Is it OK ?

No, it is not.

Another take on this: Can't we free all the page_cgroup structure before
we actually start removing the sections ? If we do this, we would be
basically left with no problem at all, since when your code starts
running we would no longer have any page_cgroup allocated.

All you have to guarantee is that it happens after the memory block is
already isolated and allocations no longer can reach it.

What do you think ?