Re: [RFC][PATCH] x86: make text_poke() atomic

Mathieu Desnoyers
2009-03-02T17:19:50Z

* Masami Hiramatsu (mhiramat@redhat.com) wrote:
> Masami Hiramatsu wrote:
>> Mathieu Desnoyers wrote:
>>> * Masami Hiramatsu (mhiramat@redhat.com) wrote:
>>>> Mathieu Desnoyers wrote:
>>>>> * Masami Hiramatsu (mhiramat@redhat.com) wrote:
>>>>>> Steven Rostedt wrote:
>>>>>>> On Mon, 23 Feb 2009, Mathieu Desnoyers wrote:
>>>>>>>>> Hmm, lets see. I simply set a bit in the PTE mappings.
>>>>>>>>> There's not many, since a lot are 2M pages, for x86_64.
>>>>>>>>> Call stop_machine, and now I can modify 1 or 20,000
>>>>>>>>> locations. Set the PTE bit back. Note, the changing of
>>>>>>>>> the bits are only done when CONFIG_DEBUG_RODATA is set.
>>>>>>>>>
>>>>>>>>> text_poke requires allocating a page. Map the page into
>>>>>>>>> memory. Set up a break point.
>>>>>>>> text_poke does not _require_ a break point. text_poke can work with
>>>>>>>> stop_machine.
>>>>>>> It can? Doesn't text_poke require allocating pages? The code
>>>>>>> called by stop_machine is all atomic. vmap does not give an
>>>>>>> option to allocate with GFP_ATOMIC.
>>>>>> Hi,
>>>>>>
>>>>>> With my patch, text_poke() never allocate pages any more :)
>>>>>>
>>>>>> BTW, IMHO, both of your methods are useful and have trade-off.
>>>>>>
>>>>>> ftrace wants to change massive amount of code at once. If we do
>>>>>> that with text_poke(), we have to map/unmap pages each time and
>>>>>> it will take a long time -- might be longer than one stop_machine_run().
>>>>>>
>>>>>> On the other hand, text_poke() user like as kprobes and tracepoints,
>>>>>> just want to change a few amount of code at once, and it will be
>>>>>> added/removed incrementally. If we do that with stop_machine_run(),
>>>>>> we'll be annoyed by frequent machine stops.(Moreover, kprobes uses
>>>>>> breakpoint, so it doesn't need stop_machine_run())
>>>>>>
>>>>> Hi Masami,
>>>>>
>>>>> Is this text_poke version executable in atomic context ? If yes, then
>>>>> that would be good to add a comment saying it. Please see below for
>>>>> comments.
>>>> Thank you for comments!
>>>> I think it could be. ah, spin_lock might be changed to spin_lock_irqsave()...
>>>>
>>> You are right. If we plan to execute this in both atomic and non-atomic
>>> context, spin_lock_irqsave would make sure we are always busy-looping
>>> with interrupts off.
>>
>> Oops, when I tested spin_lock_irqsave, it caused warnings.
>>
>> ------------[ cut here ]------------
>> WARNING: at /home/mhiramat/ksrc/linux-2.6/kernel/smp.c:329 smp_call_function_man
>> y+0x37/0x1c9()
>> Hardware name: Precision WorkStation T5400
>> Modules linked in:
>> Pid: 1, comm: swapper Tainted: G W 2.6.29-rc6 #16
>> Call Trace:
>> [<c042f7b1>] warn_slowpath+0x71/0xa8
>> [<c044dccb>] ? trace_hardirqs_on_caller+0x18/0x145
>> [<c06dc42f>] ? _spin_unlock_irq+0x22/0x2f
>> [<c044efc9>] ? print_lock_contention_bug+0x14/0xd7
>> [<c044efc9>] ? print_lock_contention_bug+0x14/0xd7
>> [<c044cfbb>] ? trace_hardirqs_off_caller+0x18/0xa3
>> [<c045383b>] smp_call_function_many+0x37/0x1c9
>> [<c04138fc>] ? do_flush_tlb_all+0x0/0x3c
>> [<c04138fc>] ? do_flush_tlb_all+0x0/0x3c
>> [<c04539e9>] smp_call_function+0x1c/0x23
>> [<c0433ee1>] on_each_cpu+0xf/0x3a
>> [<c04138c6>] flush_tlb_all+0x14/0x16
>> [<c04946f7>] unmap_kernel_range+0xf/0x11
>> [<c06dd78a>] text_poke+0xf1/0x12c
>>
>> unmap_kernel_range() requires irq enabled, but spin_lock_irqsave
>> (and stop_machine too)disables irq. so we have to solve this issue.
>>
>> I have some ideas:
>>
>> - export(just remove static) vunmap_page_range() and don't use
>> flush_tlb_all().
>> Because this vm_area are not used by other cpus, we don't need
>> to flush TLB of all cpus.
>>
>> - make unmap_kernel_range_local() function.
>>
>> - extend kmap_atomic_prot() to map lowmem page when the 'prot'
>> is different.
>>
>
> I updated my patch based on the first idea.
> I also checked that text_poke() can be called from stop_machine()
> with this patch.
> ---
>
> Use map_vm_area() instead of vmap() in text_poke() for avoiding page allocation
> and delayed unmapping, and call vunmap_page_range() and local_flush_tlb()
> directly because this mapping is temporary and local.
>
> At the result of above change, text_poke() becomes atomic and can be called
> from stop_machine() etc.
>
> Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
> Cc: Nick Piggin <npiggin@suse.de>
> ---
> arch/x86/include/asm/alternative.h | 1 +
> arch/x86/kernel/alternative.c | 36 +++++++++++++++++++++++++++++-------
> include/linux/vmalloc.h | 1 +
> init/main.c | 3 +++
> mm/vmalloc.c | 2 +-
> 5 files changed, 35 insertions(+), 8 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
> @@ -12,6 +12,7 @@
> #include <asm/nmi.h>
> #include <asm/vsyscall.h>
> #include <asm/cacheflush.h>
> +#include <asm/tlbflush.h>
> #include <asm/io.h>
>
> #define MAX_PATCH_LEN (255-1)
> @@ -485,6 +486,16 @@ 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 +512,9 @@ 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 +527,22 @@ 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);
> - local_irq_save(flags);
> + spin_lock_irqsave(&text_poke_lock, flags);
> + vma = text_poke_area[nr_pages-1];
> + ret = map_vm_area(vma, PAGE_KERNEL, &pgp);
> + BUG_ON(ret);
> + vaddr = vma->addr;
> memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
> - local_irq_restore(flags);
> - vunmap(vaddr);
> + /* Ported from unmap_kernel_range() */
> + flush_cache_vunmap((unsigned long)vma->addr, (unsigned long)vma->size);
> + vunmap_page_range((unsigned long)vma->addr,
> + (unsigned long)vma->addr + (unsigned long)vma->size);
> + /*
> + * Since this mapping is temporary, local and protected by spinlock,
> + * we just need to flush TLB on local processor.
> + */
> + local_flush_tlb();
> + spin_unlock_irqrestore(&text_poke_lock, flags);
> 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

All good, except this above. There should be an empty text_poke_init()
in some header file, and an implementation for the X86 arch rather than
a ifdef in init/main.c.

Mathieu

> check_bugs();
>
> acpi_early_init(); /* before LAPIC and SMP init */
> Index: linux-2.6/mm/vmalloc.c
> ===================================================================
> --- linux-2.6.orig/mm/vmalloc.c
> +++ linux-2.6/mm/vmalloc.c
> @@ -71,7 +71,7 @@ static void vunmap_pud_range(pgd_t *pgd,
> } while (pud++, addr = next, addr != end);
> }
>
> -static void vunmap_page_range(unsigned long addr, unsigned long end)
> +void vunmap_page_range(unsigned long addr, unsigned long end)
> {
> pgd_t *pgd;
> unsigned long next;
> Index: linux-2.6/include/linux/vmalloc.h
> ===================================================================
> --- linux-2.6.orig/include/linux/vmalloc.h
> +++ linux-2.6/include/linux/vmalloc.h
> @@ -96,6 +96,7 @@ extern struct vm_struct *remove_vm_area(
> extern int map_vm_area(struct vm_struct *area, pgprot_t prot,
> struct page ***pages);
> extern void unmap_kernel_range(unsigned long addr, unsigned long size);
> +extern void vunmap_page_range(unsigned long addr, unsigned long end);
>
> /* Allocate/destroy a 'vmalloc' VM area. */
> extern struct vm_struct *alloc_vm_area(size_t size);
>
> --
> Masami Hiramatsu
>
> Software Engineer
> Hitachi Computer Products (America) Inc.
> Software Solutions Division
>
> e-mail: mhiramat@redhat.com
>
>

--
Mathieu Desnoyers
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