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    SubjectRe: Fw: potential /dev/urandom scalability improvement
    David Mosberger <davidm@napali.hpl.hp.com> wrote:
    >
    > >>>>> On Fri, 26 Mar 2004 11:06:19 +0000, Dave Jones <davej@redhat.com> said:
    >
    > Dave> On Thu, Mar 25, 2004 at 06:00:14PM -0800, Andrew Morton wrote:
    > >> +static inline void prefetch_range(void *addr, size_t len) +{
    > >> +#ifdef ARCH_HAS_PREFETCH + char *cp; + char *end = addr + len; +
    > >> + for (cp = addr; cp < end; cp += PREFETCH_STRIDE) +
    > >> prefetch(cp); +#endif +} + #endif
    >
    > Dave> I think this may be dangerous on some CPUs, if may prefetch
    > Dave> past the end of the buffer. Ie, if PREFETCH_STRIDE was 32, and
    > Dave> len was 65, we'd end up prefetching 65->97. As well as being
    > Dave> wasteful to cachelines, this can crash if theres for eg
    > Dave> nothing mapped after the next page boundary.
    >
    > Huh? It only ever prefetches addresses that are _within_ the
    > specified buffer. Of course it will prefetch entire cachelines, but I
    > hope you're not worried about cachlines crossing page-boundaries! ;-))
    >

    But the start address which is fed into prefetch_range() may not be
    cacheline-aligned. So if appropriately abused, a prefetch_range() could
    wander off the end of the user's buffer and into a new page.

    I think this gets it right, but I probably screwed something up.

    static inline void prefetch_range(void *addr, size_t len)
    {
    #ifdef ARCH_HAS_PREFETCH
    char *cp;
    unsigned long end;

    end = ((unsigned long)addr + len + PREFETCH_STRIDE - 1);
    end &= ~(PREFETCH_STRIDE - 1);

    for (cp = addr; cp < (char *)end; cp += PREFETCH_STRIDE)
    prefetch(cp);
    #endif
    }

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