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    SubjectRe: Reduce struct page by 8 bytes on 64bit
    On Wed, Apr 16, 2003 at 01:35:39PM -0700, Andrew Morton wrote:
    > Matthew Wilcox <willy@debian.org> wrote:
    > >
    > > Jacob's would break if we hashed to different spinlocks. But we don't, we
    > > shift right by 8, so we get the same spinlock for atomic things that are on
    > > the same "cacheline" (i think PA cachelines are actually 64 or 128 bytes,
    > > depending on model).
    > >
    >
    > Are you prepared to cast this in stone?

    I think so. It makes sense to me that we lock an entire cacheline for
    this kind of thing. Indeed, locking a smaller amount would probably break
    other stuff. Remember set_bit() et al take a pointer to an unsigned long...
    but can take a bit number > number of bits in an unsigned long. If anything,
    we should maybe expand the range covered by a single lock to a larger amount
    than 256 bytes. How big are ext2 bitmaps, for example?

    --
    "It's not Hollywood. War is real, war is primarily not about defeat or
    victory, it is about death. I've seen thousands and thousands of dead bodies.
    Do you think I want to have an academic debate on this subject?" -- Robert Fisk
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