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

[Matt Mackall <mpm@selenic.com>]

On Wed, 2008-06-11 at 13:11 -0700, Andrew Morton wrote:
> On Wed, 11 Jun 2008 14:53:00 -0500
> Matt Mackall <mpm@selenic.com> wrote:
> 
> > [adding Hugh to the cc:]
> > 
> > On Wed, 2008-06-11 at 12:37 -0700, Andrew Morton wrote:
> > > On Wed, 11 Jun 2008 11:02:31 -0700
> > > Dave Hansen <dave@linux.vnet.ibm.com> wrote:
> > > 
> > > > 
> > > > We were walking right into huge page areas in the pagemap
> > > > walker, and calling the pmds pmd_bad() and clearing them.
> > > > 
> > > > That leaked huge pages.  Bad.
> > > > 
> > > > This patch at least works around that for now.  It ignores
> > > > huge pages in the pagemap walker for the time being, and
> > > > won't leak those pages.
> > > > 
> > > 
> > > I don't get it.   Why can't we just stick a
> > > 
> > > 	if (pmd_huge(pmd))
> > > 		continue;
> > > 
> > > into pagemap_pte_range()?  Or something like that.
> > 
> > That's certainly what you'd hope to be able to do, yes.
> > 
> > If I recall the earlier discussion, some arches with huge pages can only
> > identify them via a VMA. Obviously, any arch with hardware that walks
> > our pagetables directly must be able to identify huge pages directly
> > from those tables, but I think PPC and a couple others that don't have
> > hardware TLB fill fail to store such a bit in the tables at all.
> 
> Really?  There already a couple of pmd_huge() tests in mm/memory.c and
> Rik's access_process_vm-device-memory-infrastructure.patch adds another
> one.

Quoting Hugh:

i>>?A pmd_huge(*pmd) test is tempting, but it only ever says "yes" on x86:
we've carefully left it undefined what happens to the pgd/pud/pmd/pte
hierarchy in the general arch case, once you're amongst hugepages.

-- 
Mathematics is the supreme nostalgia of our time.

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