Re: [PATCH 1/3] Revert "thp: make MADV_HUGEPAGE check for mm->def_flags"

[Alex Thorlton <athorlton@sgi.com>]

On Fri, Jan 31, 2014 at 02:52:24PM -0800, Andrew Morton wrote:
> On Fri, 31 Jan 2014 12:23:43 -0600 Alex Thorlton <athorlton@sgi.com> wrote:
> 
> > This reverts commit 8e72033f2a489b6c98c4e3c7cc281b1afd6cb85cm, and adds
> 
> 'm' is not a hex digit ;)

My mistake!  Sorry about that.

> > in code to fix up any issues caused by the revert.
> > 
> > The revert is necessary because hugepage_madvise would return -EINVAL
> > when VM_NOHUGEPAGE is set, which will break subsequent chunks of this
> > patch set.
> 
> This is a bit skimpy.  Why doesn't the patch re-break kvm-on-s390?
> 
> it would be nice to have a lot more detail here, please.  What was the
> intent of 8e72033f2a48, how this patch retains 8e72033f2a48's behavior,
> etc.

I'm actually not too sure about this, off hand.  I just know that we
couldn't have it in there because of the check for VM_NOHUGEPAGE.  The
s390 guys approved the revert, as long as we added in the following
piece:

> > --- a/arch/s390/mm/pgtable.c
> > +++ b/arch/s390/mm/pgtable.c
> > @@ -504,6 +504,9 @@ static int gmap_connect_pgtable(unsigned long address, unsigned long segment,
> >  	if (!pmd_present(*pmd) &&
> >  	    __pte_alloc(mm, vma, pmd, vmaddr))
> >  		return -ENOMEM;
> > +	/* large pmds cannot yet be handled */
> > +	if (pmd_large(*pmd))
> > +		return -EFAULT;
> 
> This bit wasn't in 8e72033f2a48.

I added the fix-up code in with the revert, so that it would all be in
one place; wasn't sure what the standard was for this sort of thing.  If
it's preferable to see this code in a separate patch, that's easy enough
to do.

I'll look into exactly what the original commit was intended to do, and
get a better description of what's going on here.  Let me know if I
should split the two changes into separate patches.

- Alex

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