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

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Date:	Mon, 3 Feb 2014 11:14:12 -0600
From:	Alex Thorlton <athorlton@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>,
	Gerald Schaefer <gerald.schaefer@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Sasha Levin <sasha.levin@...cle.com>, linux390@...ibm.com,
	linux-s390@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/3] Revert "thp: make MADV_HUGEPAGE check for
 mm->def_flags"

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@....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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