Re: Reduce struct page by 8 bytes on 64bit

[Matthew Wilcox <willy@debian.org>]

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?

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