ioremap uses __get_vm_area_node which sets alignment to fls of requested size.
I couldn't find any reason for such big align. Does it decrease TLB misses?
I tested it on custom ARM board with 200+ Mb of ioremap and it works.
What am I missing?
Alignment restriction for ioremap region was introduced with the commit:
> Author: James Bottomley <jejb@mulgrave.(none)>
> Date: Wed Jun 30 11:11:14 2004 -0500
>
> Add vmalloc alignment constraints
>
> vmalloc is used by ioremap() to get regions for
> remapping I/O space. To feed these regions back
> into a __get_free_pages() type memory allocator,
> they are expected to have more alignment than
> get_vm_area() proves. So add additional alignment
> constraints for VM_IOREMAP.
>
> Signed-off-by: James Bottomley <[email protected]>
Cc: Russell King <[email protected]>
Cc: Guan Xuetao <[email protected]>
Cc: Nicolas Pitre <[email protected]>
Cc: James Bottomley <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Dyasly Sergey <[email protected]>
Signed-off-by: Dmitry Safonov <[email protected]>
---
arch/arm/include/asm/memory.h | 5 -----
arch/unicore32/include/asm/memory.h | 5 -----
include/linux/vmalloc.h | 8 --------
mm/vmalloc.c | 2 --
4 files changed, 20 deletions(-)
diff --git a/arch/arm/include/asm/memory.h b/arch/arm/include/asm/memory.h
index 184def0..b333245 100644
--- a/arch/arm/include/asm/memory.h
+++ b/arch/arm/include/asm/memory.h
@@ -78,11 +78,6 @@
*/
#define XIP_VIRT_ADDR(physaddr) (MODULES_VADDR + ((physaddr) & 0x000fffff))
-/*
- * Allow 16MB-aligned ioremap pages
- */
-#define IOREMAP_MAX_ORDER 24
-
#else /* CONFIG_MMU */
/*
diff --git a/arch/unicore32/include/asm/memory.h b/arch/unicore32/include/asm/memory.h
index debafc4..ffae189 100644
--- a/arch/unicore32/include/asm/memory.h
+++ b/arch/unicore32/include/asm/memory.h
@@ -46,11 +46,6 @@
#define MODULES_END (PAGE_OFFSET)
/*
- * Allow 16MB-aligned ioremap pages
- */
-#define IOREMAP_MAX_ORDER 24
-
-/*
* Physical vs virtual RAM address space conversion. These are
* private definitions which should NOT be used outside memory.h
* files. Use virt_to_phys/phys_to_virt/__pa/__va instead.
diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
index b87696f..2f428e8 100644
--- a/include/linux/vmalloc.h
+++ b/include/linux/vmalloc.h
@@ -18,14 +18,6 @@ struct vm_area_struct; /* vma defining user mapping in mm_types.h */
#define VM_UNINITIALIZED 0x00000020 /* vm_struct is not fully initialized */
/* bits [20..32] reserved for arch specific ioremap internals */
-/*
- * Maximum alignment for ioremap() regions.
- * Can be overriden by arch-specific value.
- */
-#ifndef IOREMAP_MAX_ORDER
-#define IOREMAP_MAX_ORDER (7 + PAGE_SHIFT) /* 128 pages */
-#endif
-
struct vm_struct {
struct vm_struct *next;
void *addr;
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 39c3388..c4f480dd 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1313,8 +1313,6 @@ static struct vm_struct *__get_vm_area_node(unsigned long size,
struct vm_struct *area;
BUG_ON(in_interrupt());
- if (flags & VM_IOREMAP)
- align = 1ul << clamp(fls(size), PAGE_SHIFT, IOREMAP_MAX_ORDER);
size = PAGE_ALIGN(size);
if (unlikely(!size))
--
1.9.1
On Tuesday 23 December 2014 13:00:13 Dmitry Safonov wrote:
> ioremap uses __get_vm_area_node which sets alignment to fls of requested size.
> I couldn't find any reason for such big align. Does it decrease TLB misses?
> I tested it on custom ARM board with 200+ Mb of ioremap and it works.
> What am I missing?
The alignment was originally introduced in this commit:
commit ff0daca525dde796382b9ccd563f169df2571211
Author: Russell King <[email protected]>
Date: Thu Jun 29 20:17:15 2006 +0100
[ARM] Add section support to ioremap
Allow section mappings to be setup using ioremap() and torn down
with iounmap(). This requires additional support in the MM
context switch to ensure that mappings are properly synchronised
when mapped in.
Based an original implementation by Deepak Saxena, reworked and
ARMv6 support added by rmk.
Signed-off-by: Russell King <[email protected]>
and then later extended to 16MB supersection mappings, which indeed
is used to reduce TLB pressure.
I don't see any downsides to it, why change it?
Arnd