On Mon, 28 Feb 2011 10:53:16 +0100
Michal Hocko [off-list ref] wrote:

On Mon 28-02-11 18:23:22, KAMEZAWA Hiroyuki wrote:
[...]

quoted

quoted

From 84a9555741b59cb2a0a67b023e4bd0f92c670ca1 Mon Sep 17 00:00:00 2001
From: Michal Hocko <redacted>
Date: Thu, 24 Feb 2011 11:25:44 +0100
Subject: [PATCH] page_cgroup: Reduce allocation overhead for page_cgroup array for CONFIG_SPARSEMEM

Currently we are allocating a single page_cgroup array per memory
section (stored in mem_section->base) when CONFIG_SPARSEMEM is selected.
This is correct but memory inefficient solution because the allocated
memory (unless we fall back to vmalloc) is not kmalloc friendly:
        - 32b - 16384 entries (20B per entry) fit into 327680B so the
          524288B slab cache is used
        - 32b with PAE - 131072 entries with 2621440B fit into 4194304B
        - 64b - 32768 entries (40B per entry) fit into 2097152 cache

This is ~37% wasted space per memory section and it sumps up for the
whole memory. On a x86_64 machine it is something like 6MB per 1GB of
RAM.

We can reduce the internal fragmentation by using alloc_pages_exact
which allocates PAGE_SIZE aligned blocks so we will get down to <4kB
wasted memory per section which is much better.

We still need a fallback to vmalloc because we have no guarantees that
we will have a continuous memory of that size (order-10) later on during
the hotplug events.

Signed-off-by: Michal Hocko <redacted>
CC: Dave Hansen <redacted>
CC: KAMEZAWA Hiroyuki <redacted>

Acked-by: KAMEZAWA Hiroyuki <redacted>

Thanks. I will repost it with Andrew in the CC.

quoted

But...nitpick, it may be from my fault..

[...]

quoted

quoted

+static void free_page_cgroup(void *addr)
+{
+	if (is_vmalloc_addr(addr)) {
+		vfree(addr);
+	} else {
+		struct page *page = virt_to_page(addr);
+		if (!PageReserved(page)) { /* Is bootmem ? */

I think we never see PageReserved if we just use alloc_pages_exact()/vmalloc().

I have checked that and we really do not (unless I am missing some
subtle side effects). Anyway, I think we still should at least BUG_ON on
that.

quoted

Maybe my old patch was not enough and this kind of junks are remaining in
the original code.

Should I incorporate it into the patch. I think that a separate one
would be better for readability.

---
From e7a897a42b526620eb4afada2d036e1c9ff9e62a Mon Sep 17 00:00:00 2001
From: Michal Hocko <redacted>
Date: Mon, 28 Feb 2011 10:43:12 +0100
Subject: [PATCH] page_cgroup array is never stored on reserved pages

KAMEZAWA Hiroyuki noted that free_pages_cgroup doesn't have to check for
PageReserved because we never store the array on reserved pages
(neither alloc_pages_exact nor vmalloc use those pages).

So we can replace the check by a BUG_ON.

Signed-off-by: Michal Hocko <redacted>
CC: KAMEZAWA Hiroyuki <redacted>

Thank you.
Acked-by: KAMEZAWA Hiroyuki <redacted>

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