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    SubjectRe: [PATCH 4/6] ftrace, x86: make kernel text writable only for conversions
    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.


    Thanks,

    >
    > Having spinlocks taken in _both_ interrupts on and off contexts leads to
    > higher interrupt latencies when the interrupt-off waits for an
    > interrupt-on thread.
    >
    >
    >>>> Thank you,
    >>>>
    >>> [...]
    >>>> Use map_vm_area() instead of vmap() in text_poke() for avoiding page allocation
    >>>> and delayed unmapping.
    >>>>
    >>>> Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
    >>>> ---
    >>>> 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);
    >>> Why is this text_poke_area[1] 2 * PAGE_SIZE in size ? I would have
    >>> thought that text_poke_area[0] would be PAGE_SIZE, text_poke_area[1]
    >>> also be PAGE_SIZE, and that the sum of both would be 2 * PAGE_SIZE..
    >> Unfortunately, current map_vm_area() tries to map the size of vm_area,
    >> this means, you can't use 2page-size vm_area for mapping just 1 page...
    >> (or maybe, we can set pages[1] = pages[0] when 2nd page doesn't exist)
    >>
    >
    > OK, given we sometimes have to map only a single page (e.g. at the end
    > of a text section), we really need both 1 and 2 pages mapping. So I
    > think you solution is good.
    >
    >>>> + 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;
    >>> Hrm, why do you need **pgp ? Could you simply pass &pages to map_vm_area ?
    >> As you know, pages means just the address(value) of an array, so you can't
    >> get the address of the address...(pages and &pages are same.)
    >>
    >> int array[2];
    >> printf("%p, %p",array, &array);
    >>
    >> please try it :)
    >>
    >> And actually, map_vm_area() requires the address of a pointer.
    >
    > Ah yes, thanks for the explanation.
    >
    > After changing the spinlock/irqsave, I think that patch would be good to
    > merge. And then Steve could use text_poke within stop_machine if he
    > likes.
    >
    > Mathieu
    >
    >> ---
    >> int map_vm_area(struct vm_struct *area, pgprot_t prot, struct page ***pages)
    >> {
    >> unsigned long addr = (unsigned long)area->addr;
    >> unsigned long end = addr + area->size - PAGE_SIZE;
    >> int err;
    >>
    >> err = vmap_page_range(addr, end, prot, *pages);
    >> if (err > 0) {
    >> *pages += err;
    >> ^^^^^^^^^^^^^^ Here, it tries to add err(=number of mapped pages)
    >> to the pages pointer!
    >> err = 0;
    >> }
    >>
    >> return err;
    >> }
    >> ---
    >>
    >>
    >>> Thanks,
    >>>
    >>> Mathieu
    >>>
    >>>> + 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. */
    >>>> @@ -528,3 +543,4 @@ void *__kprobes text_poke(void *addr, co
    >>>> BUG_ON(((char *)addr)[i] != ((char *)opcode)[i]);
    >>>> return addr;
    >>>> }
    >>>> +EXPORT_SYMBOL_GPL(text_poke);
    >>>> 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@redhat.com
    >>
    >

    --
    Masami Hiramatsu

    Software Engineer
    Hitachi Computer Products (America) Inc.
    Software Solutions Division

    e-mail: mhiramat@redhat.com


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