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




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