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    SubjectRe: cpumask: fix compat getaffinity
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    On Sat, 8 May 2010 06:41:08 pm Arnd Bergmann wrote:
    > On Saturday 08 May 2010 10:30:47 Rusty Russell wrote:
    > >
    > > On Fri, 7 May 2010 10:15:49 pm Arnd Bergmann wrote:
    > > > Commit a45185d2d "cpumask: convert kernel/compat.c" broke
    > > > libnuma, which abuses sched_getaffinity to find out NR_CPUS
    > > > in order to parse /sys/devices/system/node/node*/cpumap.
    > > >
    > > > On NUMA systems with less than 32 possibly CPUs, the
    > > > current compat_sys_sched_getaffinity now returns '4'
    > > > instead of the actual NR_CPUS/8, which makes libnuma
    > > > bail out when parsing the cpumap.
    > >
    > > Really? AFAICT the cpumap is printed using nr_cpu_ids too. Can you
    > > give an example of what cpumap is on this system?
    >
    > On Ken's PS3 running the Fedora 12 kernel (2.6.32-something), the output
    > from my memory is 00000000,00000000,00000000,00000003\n. NR_CPUs
    > is 128, nr_cpu_ids is most likely 2.

    Ah, I see. CONFIG_CPUMASK_OFFSTACK=n.

    The nr_cpumask_bits is really an optimization for cpumask_first etc: making
    it a constant (NR_CPUS) rather than a variable (nr_cpu_ids) for small NR_CPUS
    makes it slightly faster.

    However, we also use this for scnprintf et al, revealing this inconsistency.
    Your patch would break libnuma on CONFIG_CPUMASK_OFFSTACK=y.

    How's this?

    cpumask: use nr_cpu_ids for printing and parsing cpumasks

    Commit a45185d2d "cpumask: convert kernel/compat.c" broke
    libnuma, which abuses sched_getaffinity to find out NR_CPUS
    in order to parse /sys/devices/system/node/node*/cpumap.

    However, the result now returned reflects nr_cpu_ids, and
    cpumask_scnprintf et al. use nr_cpumask_bits which is NR_CPUS (for
    CONFIG_CPUMASK_OFFSTACK=n) or nr_cpu_ids (for
    CONFIG_CPUMASK_OFFSTACK=y).

    We should use nr_cpu_ids consistently.

    Reported-by: Arnd Bergmann <arnd@arndb.de>
    Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
    Cc: stable@kernel.org

    diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
    --- a/include/linux/cpumask.h
    +++ b/include/linux/cpumask.h
    @@ -529,7 +529,7 @@ static inline void cpumask_copy(struct c
    static inline int cpumask_scnprintf(char *buf, int len,
    const struct cpumask *srcp)
    {
    - return bitmap_scnprintf(buf, len, cpumask_bits(srcp), nr_cpumask_bits);
    + return bitmap_scnprintf(buf, len, cpumask_bits(srcp), nr_cpu_ids);
    }

    /**
    @@ -543,7 +543,7 @@ static inline int cpumask_scnprintf(char
    static inline int cpumask_parse_user(const char __user *buf, int len,
    struct cpumask *dstp)
    {
    - return bitmap_parse_user(buf, len, cpumask_bits(dstp), nr_cpumask_bits);
    + return bitmap_parse_user(buf, len, cpumask_bits(dstp), nr_cpu_ids);
    }

    /**
    @@ -558,8 +558,7 @@ static inline int cpumask_parse_user(con
    static inline int cpulist_scnprintf(char *buf, int len,
    const struct cpumask *srcp)
    {
    - return bitmap_scnlistprintf(buf, len, cpumask_bits(srcp),
    - nr_cpumask_bits);
    + return bitmap_scnlistprintf(buf, len, cpumask_bits(srcp), nr_cpu_ids);
    }

    /**
    @@ -572,7 +571,7 @@ static inline int cpulist_scnprintf(char
    */
    static inline int cpulist_parse(const char *buf, struct cpumask *dstp)
    {
    - return bitmap_parselist(buf, cpumask_bits(dstp), nr_cpumask_bits);
    + return bitmap_parselist(buf, cpumask_bits(dstp), nr_cpu_ids);
    }

    /**

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