Re: irq-disabled vs vmap vs text_poke
From: Masami Hiramatsu
Date: Mon Feb 16 2009 - 21:01:32 EST
Mathieu Desnoyers wrote:
* Nick Piggin (npiggin@xxxxxxx) wrote:
On Mon, Feb 16, 2009 at 10:04:43AM -0500, Masami Hiramatsu wrote:
BTW, what about using map_vm_area() in text_poke() instead of vmap()?
Since text_poke() just maps text pages to alias pages temporarily,
I think we don't need to use delayed vunmap().
[...]
Here is the patch which replace v(un)map with (un)map_vm_area.
I don't quite understand the point of this... delayed vunmap() is
just an implementation detail of vmap subsystem. Callers should not
have to care.
AFAIK, map_vm_area/unmap_vm_area is faster than vmap/vunmap. This is
the point of this patch. Masami, could you provide a quick benchmark of
text_poke()/seconds before and after this optimization is applied to
confirm this ?
Sure, here is the result of calling text_poke() 2^14 times.
<Without this patch>
Total: 3634133356(cycles), 221809(cycles/text_poke)
Total: 3699532690(cycles), 225801(cycles/text_poke)
Total: 3249855588(cycles), 198355(cycles/text_poke)
<With this patch>
Total: 483467579(cycles), 29508(cycles/text_poke)
Total: 497441301(cycles), 30361(cycles/text_poke)
Total: 497604548(cycles), 30371(cycles/text_poke)
BTW, this is not only for performance, but also simplicity and its need.
Vmap may allocate new vm_area. However, since text_poke() just needs to
map pages temporarily (yeah, very short time), we don't want to call
kmalloc or any other memory allocators.
And since text_poke() makes WRITABLE aliases of READ-ONLY pages, we
want to purge these pages ASAP.
So, I think just reserving a small vm_area for text_poke() and
reusing it is enough.
Thanks,
Thanks,
Mathieu
Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
---
arch/x86/include/asm/alternative.h | 1 +
arch/x86/kernel/alternative.c | 25 ++++++++++++++++++++-----
init/main.c | 3 +++
3 files changed, 24 insertions(+), 5 deletions(-)
Index: linux-2.6/arch/x86/include/asm/alternative.h
===================================================================
--- linux-2.6.orig/arch/x86/include/asm/alternative.h
+++ linux-2.6/arch/x86/include/asm/alternative.h
@@ -177,6 +177,7 @@ extern void add_nops(void *insns, unsign
* The _early version expects the memory to already be RW.
*/
+extern void text_poke_init(void);
extern void *text_poke(void *addr, const void *opcode, size_t len);
extern void *text_poke_early(void *addr, const void *opcode, size_t len);
Index: linux-2.6/arch/x86/kernel/alternative.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/alternative.c
+++ linux-2.6/arch/x86/kernel/alternative.c
@@ -485,6 +485,16 @@ void *text_poke_early(void *addr, const
return addr;
}
+static struct vm_struct *text_poke_area[2];
+static DEFINE_SPINLOCK(text_poke_lock);
+
+void __init text_poke_init(void)
+{
+ text_poke_area[0] = get_vm_area(PAGE_SIZE, VM_ALLOC);
+ text_poke_area[1] = get_vm_area(2 * PAGE_SIZE, VM_ALLOC);
+ BUG_ON(!text_poke_area[0] || !text_poke_area[1]);
+}
+
/**
* text_poke - Update instructions on a live kernel
* @addr: address to modify
@@ -501,8 +511,9 @@ void *__kprobes text_poke(void *addr, co
unsigned long flags;
char *vaddr;
int nr_pages = 2;
- struct page *pages[2];
- int i;
+ struct page *pages[2], **pgp = pages;
+ int i, ret;
+ struct vm_struct *vma;
if (!core_kernel_text((unsigned long)addr)) {
pages[0] = vmalloc_to_page(addr);
@@ -515,12 +526,16 @@ void *__kprobes text_poke(void *addr, co
BUG_ON(!pages[0]);
if (!pages[1])
nr_pages = 1;
- vaddr = vmap(pages, nr_pages, VM_MAP, PAGE_KERNEL);
- BUG_ON(!vaddr);
+ spin_lock(&text_poke_lock);
+ vma = text_poke_area[nr_pages-1];
+ ret = map_vm_area(vma, PAGE_KERNEL, &pgp);
+ BUG_ON(ret);
+ vaddr = vma->addr;
local_irq_save(flags);
memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
local_irq_restore(flags);
- vunmap(vaddr);
+ unmap_kernel_range((unsigned long)vma->addr, (unsigned
long)vma->size);
+ spin_unlock(&text_poke_lock);
sync_core();
/* Could also do a CLFLUSH here to speed up CPU recovery; but
that causes hangs on some VIA CPUs. */
Index: linux-2.6/init/main.c
===================================================================
--- linux-2.6.orig/init/main.c
+++ linux-2.6/init/main.c
@@ -676,6 +676,9 @@ asmlinkage void __init start_kernel(void
taskstats_init_early();
delayacct_init();
+#ifdef CONFIG_X86
+ text_poke_init();
+#endif
check_bugs();
acpi_early_init(); /* before LAPIC and SMP init */
--
Masami Hiramatsu
Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division
e-mail: mhiramat@xxxxxxxxxx
--
Masami Hiramatsu
Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division
e-mail: mhiramat@xxxxxxxxxx
--
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