Re: [rfc] increase struct page size?!

[Nick Piggin <npiggin@suse.de>]

On Sat, May 19, 2007 at 11:15:01AM -0700, William Lee Irwin III wrote:
> On Fri, May 18, 2007 at 11:14:26AM -0700, Christoph Lameter wrote:
> >> Right. That would simplify the calculations.
> 
> On Sat, May 19, 2007 at 03:25:30AM +0200, Nick Piggin wrote:
> > It isn't the calculations I'm worried about, although they'll get simpler
> > too. It is the cache cost.
> 
> The cache cost argument is specious. Even misaligned, smaller is
> smaller.

Of course smaller is smaller ;) Why would that make the cache cost
argument specious?


> The cache footprint reduction is merely amortized,
> probabilistic, etc.

I don't really know what you mean by this, or what part of my cache cost
argument you disagree with...

I think it is that you could construct mem_map access patterns, without
specifically looking at alignment, where a 56 byte struct page would suffer
about 75% more cache misses than a 64 byte aligned one (and you could also
get about 12% fewer cache misses with other access patterns).

I also think the kernel's mem_map access patterns would be more on the
random side, so overall would result in significantly fewer cache misses
with 64 byte aligned pages.

Which part do you disagree with?


> On Fri, May 18, 2007 at 11:14:26AM -0700, Christoph Lameter wrote:
> >> I wonder if there are other uses for the free space?
> 
> On Sat, May 19, 2007 at 03:25:30AM +0200, Nick Piggin wrote:
> > Hugh points out that we should make _count and _mapcount atomic_long_t's,
> > which would probably be a better use of the space once your vmemmap goes
> > in.
> 
> I'm not so sure about that. I doubt we have issues with that. I say

The issue is that userspace can DOS or crash the kernel by deliberately
overflowing count or mapcount.


> if there's to be padding to 64B to use the of the whole additional
> space for additional flag bits. I'm sure fs's could make good use of
> 64 spare flag bits, or whatever's left over after the VM has its fill.
> Perhaps so many spare flag bits could be used in lieu of buffer_heads.

Really? 64-bit architectures can already use about maybe 16 or 32 more
page flag bits than 32-bit architectures, and I definitely do not want
to increase the size of 32-bit struct page, so I think this wouldn't
work.


> page->virtual is the same old mistake as it was when it was removed.
> The virtual mem_map code should be used to resolve the computational

Don't get too hung up on the page->virtual thing. I'll send another
patch with atomic_t/atomic_long_t conversion.


> expense. Much the same holds for the atomic_t's; 32 + PAGE_SHIFT is
> 44 bits or more, about as much as is possible, and one reference per
> page per page is not even feasible. Full-length atomic_t's are just
> not necessary.

I don't know what your 32 + PAGE_SHIFT calculation is for, but yes you
can wrap these counters from userspace on 64-bit architectures.