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    SubjectRe: [PATCH 5/5] proc: export more page flags in /proc/kpageflags
    On Wed, Apr 29, 2009 at 05:32:44AM +0800, Andrew Morton wrote:
    > On Tue, 28 Apr 2009 09:09:12 +0800
    > Wu Fengguang <fengguang.wu@intel.com> wrote:
    >
    > > +/*
    > > + * Kernel flags are exported faithfully to Linus and his fellow hackers.
    > > + * Otherwise some details are masked to avoid confusing the end user:
    > > + * - some kernel flags are completely invisible
    > > + * - some kernel flags are conditionally invisible on their odd usages
    > > + */
    > > +#ifdef CONFIG_DEBUG_KERNEL
    > > +static inline int genuine_linus(void) { return 1; }
    >
    > Although he's a fine chap, the use of the "_linus" tag isn't terribly
    > clear (to me). I think what you're saying here is that this enables
    > kernel-developer-only features, yes?

    Yes.

    > If so, perhaps we could come up with an identifier which expresses that
    > more clearly.
    >
    > But I'd expect that everyone and all distros enable CONFIG_DEBUG_KERNEL
    > for _some_ reason, so what's the point?

    Good point! I can confirm my debian has CONFIG_DEBUG_KERNEL=Y!

    > It is preferable that we always implement the same interface for all
    > Kconfig settings. If this exposes information which is confusing or
    > not useful to end-users then so be it - we should be able to cover that
    > in supporting documentation.

    My original patch takes that straightforward manner - and I still like it.
    I would be very glad to move the filtering code from kernel to user space.

    The use of more obscure flags could be discouraged by _not_ documenting
    them. A really curious user is encouraged to refer to the code for the
    exact meaning (and perhaps become a kernel developer ;-)

    > Also, as mentioned in the other email, it would be good if we were to
    > publish a little userspace app which people can use to access this raw
    > data. We could give that application an `--i-am-a-kernel-developer'
    > option!

    OK. I'll include page-types.c in the next take.

    > > +#else
    > > +static inline int genuine_linus(void) { return 0; }
    > > +#endif
    >
    > This isn't an appropriate use of CONFIG_DEBUG_KERNEL.
    >
    > DEBUG_KERNEL is a Kconfig-only construct which is use to enable _other_
    > debugging features. The way you've used it here, if the person who is
    > configuring the kernel wants to enable any other completely-unrelated
    > debug feature, they have to enable DEBUG_KERNEL first. But when they
    > do that, they unexpectedly alter the behaviour of pagemap!
    >
    > There are two other places where CONFIG_DEBUG_KERNEL affects code
    > generation in .c files: arch/parisc/mm/init.c and
    > arch/powerpc/kernel/sysfs.c. These are both wrong, and need slapping ;)

    (add cc to related maintainers)

    CONFIG_DEBUG_KERNEL being enabled in distro kernels effectively means

    #ifdef CONFIG_DEBUG_KERNEL == #if 1

    as the following patch demos. Now it becomes obviously silly.

    diff --git a/arch/parisc/mm/init.c b/arch/parisc/mm/init.c
    index 4356ceb..59fb910 100644
    --- a/arch/parisc/mm/init.c
    +++ b/arch/parisc/mm/init.c
    @@ -368,19 +368,19 @@ static void __init setup_bootmem(void)
    request_resource(&sysram_resources[0], &pdcdata_resource);
    }

    void free_initmem(void)
    {
    unsigned long addr, init_begin, init_end;

    printk(KERN_INFO "Freeing unused kernel memory: ");

    -#ifdef CONFIG_DEBUG_KERNEL
    +#if 1
    /* Attempt to catch anyone trying to execute code here
    * by filling the page with BRK insns.
    *
    * If we disable interrupts for all CPUs, then IPI stops working.
    * Kinda breaks the global cache flushing.
    */
    local_irq_disable();

    memset(__init_begin, 0x00,
    @@ -519,19 +519,19 @@ void __init mem_init(void)
    printk(KERN_INFO "Memory: %luk/%luk available (%dk kernel code, %dk reserved, %dk data, %dk init)\n",
    (unsigned long)nr_free_pages() << (PAGE_SHIFT-10),
    num_physpages << (PAGE_SHIFT-10),
    codesize >> 10,
    reservedpages << (PAGE_SHIFT-10),
    datasize >> 10,
    initsize >> 10
    );

    -#ifdef CONFIG_DEBUG_KERNEL /* double-sanity-check paranoia */
    +#if 1 /* double-sanity-check paranoia */
    printk("virtual kernel memory layout:\n"
    " vmalloc : 0x%p - 0x%p (%4ld MB)\n"
    " memory : 0x%p - 0x%p (%4ld MB)\n"
    " .init : 0x%p - 0x%p (%4ld kB)\n"
    " .data : 0x%p - 0x%p (%4ld kB)\n"
    " .text : 0x%p - 0x%p (%4ld kB)\n",

    (void*)VMALLOC_START, (void*)VMALLOC_END,
    (VMALLOC_END - VMALLOC_START) >> 20,
    diff --git a/arch/powerpc/kernel/sysfs.c b/arch/powerpc/kernel/sysfs.c
    index f41aec8..0d54c6b 100644
    --- a/arch/powerpc/kernel/sysfs.c
    +++ b/arch/powerpc/kernel/sysfs.c
    @@ -212,19 +212,19 @@ static SYSDEV_ATTR(purr, 0600, show_purr, store_purr);
    #endif /* CONFIG_PPC64 */

    #ifdef HAS_PPC_PMC_PA6T
    SYSFS_PMCSETUP(pa6t_pmc0, SPRN_PA6T_PMC0);
    SYSFS_PMCSETUP(pa6t_pmc1, SPRN_PA6T_PMC1);
    SYSFS_PMCSETUP(pa6t_pmc2, SPRN_PA6T_PMC2);
    SYSFS_PMCSETUP(pa6t_pmc3, SPRN_PA6T_PMC3);
    SYSFS_PMCSETUP(pa6t_pmc4, SPRN_PA6T_PMC4);
    SYSFS_PMCSETUP(pa6t_pmc5, SPRN_PA6T_PMC5);
    -#ifdef CONFIG_DEBUG_KERNEL
    +#if 1
    SYSFS_PMCSETUP(hid0, SPRN_HID0);
    SYSFS_PMCSETUP(hid1, SPRN_HID1);
    SYSFS_PMCSETUP(hid4, SPRN_HID4);
    SYSFS_PMCSETUP(hid5, SPRN_HID5);
    SYSFS_PMCSETUP(ima0, SPRN_PA6T_IMA0);
    SYSFS_PMCSETUP(ima1, SPRN_PA6T_IMA1);
    SYSFS_PMCSETUP(ima2, SPRN_PA6T_IMA2);
    SYSFS_PMCSETUP(ima3, SPRN_PA6T_IMA3);
    SYSFS_PMCSETUP(ima4, SPRN_PA6T_IMA4);
    @@ -282,19 +282,19 @@ static struct sysdev_attribute classic_pmc_attrs[] = {
    static struct sysdev_attribute pa6t_attrs[] = {
    _SYSDEV_ATTR(mmcr0, 0600, show_mmcr0, store_mmcr0),
    _SYSDEV_ATTR(mmcr1, 0600, show_mmcr1, store_mmcr1),
    _SYSDEV_ATTR(pmc0, 0600, show_pa6t_pmc0, store_pa6t_pmc0),
    _SYSDEV_ATTR(pmc1, 0600, show_pa6t_pmc1, store_pa6t_pmc1),
    _SYSDEV_ATTR(pmc2, 0600, show_pa6t_pmc2, store_pa6t_pmc2),
    _SYSDEV_ATTR(pmc3, 0600, show_pa6t_pmc3, store_pa6t_pmc3),
    _SYSDEV_ATTR(pmc4, 0600, show_pa6t_pmc4, store_pa6t_pmc4),
    _SYSDEV_ATTR(pmc5, 0600, show_pa6t_pmc5, store_pa6t_pmc5),
    -#ifdef CONFIG_DEBUG_KERNEL
    +#if 1
    _SYSDEV_ATTR(hid0, 0600, show_hid0, store_hid0),
    _SYSDEV_ATTR(hid1, 0600, show_hid1, store_hid1),
    _SYSDEV_ATTR(hid4, 0600, show_hid4, store_hid4),
    _SYSDEV_ATTR(hid5, 0600, show_hid5, store_hid5),
    _SYSDEV_ATTR(ima0, 0600, show_ima0, store_ima0),
    _SYSDEV_ATTR(ima1, 0600, show_ima1, store_ima1),
    _SYSDEV_ATTR(ima2, 0600, show_ima2, store_ima2),
    _SYSDEV_ATTR(ima3, 0600, show_ima3, store_ima3),
    _SYSDEV_ATTR(ima4, 0600, show_ima4, store_ima4),
    > > +#define kpf_copy_bit(uflags, kflags, visible, ubit, kbit) \
    > > + do { \
    > > + if (visible || genuine_linus()) \
    > > + uflags |= ((kflags >> kbit) & 1) << ubit; \
    > > + } while (0);
    >
    > Did this have to be implemented as a macro?
    >
    > It's bad, because it might or might not reference its argument, so if
    > someone passes it an expression-with-side-effects, the end result is
    > unpredictable. A C function is almost always preferable if possible.

    Just tried inline function, the code size is increased slightly:

    text data bss dec hex filename
    macro 1804 128 0 1932 78c fs/proc/page.o
    inline 1828 128 0 1956 7a4 fs/proc/page.o

    So I'll keep the macro, but add brackets to make it a bit safer.

    Thanks,
    Fengguang



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