Date:	Fri, 25 Feb 2011 10:53:57 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Dave Hansen <dave@...ux.vnet.ibm.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] page_cgroup: Reduce allocation overhead for
 page_cgroup array for CONFIG_SPARSEMEM v3

On Fri 25-02-11 12:25:22, KAMEZAWA Hiroyuki wrote:
> On Thu, 24 Feb 2011 14:40:45 +0100
> Michal Hocko <mhocko@...e.cz> wrote:
> 
> > Here is the second version of the patch. I have used alloc_pages_exact
> > instead of the complex double array approach.
> > 
> > I still fallback to kmalloc/vmalloc because hotplug can happen quite
> > some time after boot and we can end up not having enough continuous
> > pages at that time. 
> > 
> > I am also thinking whether it would make sense to introduce
> > alloc_pages_exact_node function which would allocate pages from the
> > given node.
> > 
> > Any thoughts?
> 
> The patch itself is fine but please update the description.

I have updated the description but kept those parts which describe how
the memory is wasted for different configurations. Do you have any tips
how it can be improved?

> 
> But have some comments, below.
[...]
> > -/* __alloc_bootmem...() is protected by !slab_available() */
> > +static void *__init_refok alloc_mcg_table(size_t size, int nid)
> > +{
> > +	void *addr = NULL;
> > +	if((addr = alloc_pages_exact(size, GFP_KERNEL | __GFP_NOWARN)))
> > +		return addr;
> > +
> > +	if (node_state(nid, N_HIGH_MEMORY)) {
> > +		addr = kmalloc_node(size, GFP_KERNEL | __GFP_NOWARN, nid);
> > +		if (!addr)
> > +			addr = vmalloc_node(size, nid);
> > +	} else {
> > +		addr = kmalloc(size, GFP_KERNEL | __GFP_NOWARN);
> > +		if (!addr)
> > +			addr = vmalloc(size);
> > +	}
> > +
> > +	return addr;
> > +}
> 
> What is the case we need to call kmalloc_node() even when alloc_pages_exact() fails ?
> vmalloc() may need to be called when the size of chunk is larger than
> MAX_ORDER or there is fragmentation.....

I kept the original kmalloc with fallback to vmalloc because vmalloc is
more scarce resource (especially on i386 where we can have memory
hotplug configured as well).

> 
> And the function name, alloc_mcg_table(), I don't like it because this is an
> allocation for page_cgroup.
> 
> How about alloc_page_cgroup() simply ?

OK, I have no preferences for the name. alloc_page_cgroup sounds good as
well.

I have also added VM_BUG_ON(!slab_is_available()) back to the allocation
path.

Thanks for the review. The updated patch is bellow:

Changes since v2
- rename alloc_mcg_table to alloc_page_cgroup
- free__mcg_table renamed to free_page_cgroup
- get VM_BUG_ON(!slab_is_available()) back into the allocation path
--- 
>From 9b34baf57f410a628bf45f2b35b23fdf38feae79 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@...e.cz>
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 either by imeplementing 2
dimensional array and allocate kmalloc aligned sizes for each entry (as
suggested in https://lkml.org/lkml/2011/2/23/232) or allocate directly
from the buddy allocator (use alloc_pages_exact) as suggested by Dave
Hansen.

The later solution is much simpler and the internal fragmentation is
comparable (~1 page per section). The initialization is done during the
boot (unless we are doing memory hotplug) so we shouldn't have any
issues from memory fragmentation and alloc_pages_exact should succeed.

We still need a fallback to kmalloc/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 <mhocko@...e.cz>
CC: Dave Hansen <dave@...ux.vnet.ibm.com>
CC: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
---
 mm/page_cgroup.c |   62 ++++++++++++++++++++++++++++++++++-------------------
 1 files changed, 40 insertions(+), 22 deletions(-)

diff --git a/mm/page_cgroup.c b/mm/page_cgroup.c
index 5bffada..ae322dc 100644
--- a/mm/page_cgroup.c
+++ b/mm/page_cgroup.c
@@ -106,6 +106,42 @@ struct page_cgroup *lookup_page_cgroup(struct page *page)
 }
 
 /* __alloc_bootmem...() is protected by !slab_available() */
+static void *__init_refok alloc_page_cgroup(size_t size, int nid)
+{
+	void *addr = NULL;
+	if((addr = alloc_pages_exact(size, GFP_KERNEL | __GFP_NOWARN)))
+		return addr;
+
+	VM_BUG_ON(!slab_is_available());
+	if (node_state(nid, N_HIGH_MEMORY)) {
+		addr = kmalloc_node(size, GFP_KERNEL | __GFP_NOWARN, nid);
+		if (!addr)
+			addr = vmalloc_node(size, nid);
+	} else {
+		addr = kmalloc(size, GFP_KERNEL | __GFP_NOWARN);
+		if (!addr)
+			addr = vmalloc(size);
+	}
+
+	return addr;
+}
+
+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 ? */
+			if (!PageSlab(page)) {
+				size_t table_size = sizeof(struct page_cgroup) * PAGES_PER_SECTION;
+				free_pages_exact(addr, table_size);
+			} else
+				kfree(addr);
+		}
+	}
+}
+
 static int __init_refok init_section_page_cgroup(unsigned long pfn)
 {
 	struct mem_section *section = __pfn_to_section(pfn);
@@ -114,19 +150,9 @@ static int __init_refok init_section_page_cgroup(unsigned long pfn)
 	int nid, index;
 
 	if (!section->page_cgroup) {
-		nid = page_to_nid(pfn_to_page(pfn));
 		table_size = sizeof(struct page_cgroup) * PAGES_PER_SECTION;
-		VM_BUG_ON(!slab_is_available());
-		if (node_state(nid, N_HIGH_MEMORY)) {
-			base = kmalloc_node(table_size,
-				GFP_KERNEL | __GFP_NOWARN, nid);
-			if (!base)
-				base = vmalloc_node(table_size, nid);
-		} else {
-			base = kmalloc(table_size, GFP_KERNEL | __GFP_NOWARN);
-			if (!base)
-				base = vmalloc(table_size);
-		}
+		nid = page_to_nid(pfn_to_page(pfn));
+		base = alloc_page_cgroup(table_size, nid);
 		/*
 		 * The value stored in section->page_cgroup is (base - pfn)
 		 * and it does not point to the memory block allocated above,
@@ -170,16 +196,8 @@ void __free_page_cgroup(unsigned long pfn)
 	if (!ms || !ms->page_cgroup)
 		return;
 	base = ms->page_cgroup + pfn;
-	if (is_vmalloc_addr(base)) {
-		vfree(base);
-		ms->page_cgroup = NULL;
-	} else {
-		struct page *page = virt_to_page(base);
-		if (!PageReserved(page)) { /* Is bootmem ? */
-			kfree(base);
-			ms->page_cgroup = NULL;
-		}
-	}
+	free_page_cgroup(base);
+	ms->page_cgroup = NULL;
 }
 
 int __meminit online_page_cgroup(unsigned long start_pfn,
-- 
1.7.2.3

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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