Re: [v4][PATCH 2/2] fix large pages in pagemap

[Matt Mackall <mpm@selenic.com>]

On Thu, 2008-06-12 at 22:36 +0100, Hugh Dickins wrote:
> On Wed, 11 Jun 2008, Matt Mackall wrote:
> > On Wed, 2008-06-11 at 14:23 -0700, Dave Hansen wrote:
> > > On Wed, 2008-06-11 at 13:52 -0700, Andrew Morton wrote:
> > > 
> > > > Am I missing something, or is pmd_huge() a whopping big grenade for x86
> > > > developers to toss at non-x86 architectures?  It seems quite dangerous.
> > > 
> > > Yeah, it isn't really usable outside of arch code, although it kinda
> > > looks like it.
> > 
> > That begs the question: if we can't use it reliably outside of arch
> > code, why do other arches even bother defining it?
> 
> Good question.
> 
> > And the answer seems to be because of the two uses in mm/memory.c. The
> > first seems like it could be avoided with an implementation of
> > follow_huge_addr on x86.
> 
> No, I don't think we need even that: because get_user_pages avoids
> follow_page on huge vmas, and little else uses follow_page, I think
> we could delete follow_huge_addr from all architectures.
> 
> It's gives a warm glow to know that follow_page could cope with
> huge ranges if necessary, but it doesn't feel so good once you've
> been misled by pmd_huge.

Since follow_page gets a VMA, it could check.

> Incidentally, x86 turns out to have a pmd_large() too!

As does mn10300 - surely that's a mistake?

> > The second is either bogus (only works on x86)
> > or superfluous (not needed at all), no?
> 
> Not needed (checking for things that should never happen
> is nice when it's convenient, but not a reason to uglify).
> 
> On Dave's patch: yes, I too would have preferred a pmd_huge-style
> vma-less approach, but fear that doesn't work out at present.  But
> isn't the patch calling find_vma() much more often than necessary?

Well not at the level of the function in question. But it could probably
cache a current VMA value across the whole walk in the walk structure.

> Is there any architecture which can mix huge and normal pages
> within the lowest pagetable?

Not sure how that would work?

-- 
Mathematics is the supreme nostalgia of our time.

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