linux-kernel - Re: + mm-revert-thp-make-madv_hugepage-check-for-mm-def

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Date:	Wed, 26 Feb 2014 13:39:17 -0600
From:	Alex Thorlton <athorlton@....com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	viro@...iv.linux.org.uk, schwidefsky@...ibm.com,
	rientjes@...gle.com, riel@...hat.com, pbonzini@...hat.com,
	mingo@...nel.org, mgorman@...e.de, kirill.shutemov@...ux.intel.com,
	heiko.carstens@...ibm.com, hannes@...xchg.org,
	gerald.schaefer@...ibm.com, ebiederm@...ssion.com,
	aarcange@...hat.com
Subject: Re: +	mm-revert-thp-make-madv_hugepage-check-for-mm-def_flags.patch
 added	to -mm tree

On Wed, Feb 26, 2014 at 08:27:36PM +0100, Christian Borntraeger wrote:
> On 26/02/14 19:06, Oleg Nesterov wrote:
> > On 02/26, Alex Thorlton wrote:
> >>
> >> + * MADV_HUGEPAGE after PRCTL_THP_DISABLE is broken on s390 because
> >> + * qemu blindly does madvise(MADV_HUGEPAGE) after s390_enable_sie().
> >> + */
> >> +#ifdef CONFIG_S390
> >> +               if (mm_has_pgste(vma->vm_mm))
> >>                         return -EINVAL;
> >> +#endif
> > 
> > The comment is not really right...
> > 
> > And personally I think that
> > 
> > 	@@ -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;
> > 
> > change still makes sense, so that we can simply revert this s390-
> > specific hack in hugepage_madvise().
> 
> Yes, it still makes sense to cover existing THPs here.

Yes, it does.  I just snipped the chunk of the original patch that I
actually changed in my last e-mail.  I didn't intend to actually remove
the above portion in the final patch - sorry for the confusion!

> 
> 
> > I'd suggest the patch below on top of your changes, but I won't argue.

I like this approach, and your updated comment as well.  This should
probably go in as [PATCH 2/4] in my series.  Do I need to spin a v5
with this new patch included in the series, or will Andrew pull this in?

> > 
> > It would be nice to also change thp_split_mm() to not not play with
> > mm->def_flags, but I am not sure if we can do this.
> > 
> > Oleg.
> > ---
> > 
> > Subject: [PATCH] s390: make sure MADV_HUGEPAGE fails after s390_enable_sie()
> > 
> > As Christian pointed out, the recent 'Revert "thp: make MADV_HUGEPAGE
> > check for mm->def_flags"' breaks qemu, it does QEMU_MADV_HUGEPAGE for
> > all kvm pages but this doesn't work after s390_enable_sie/thp_split_mm.
> > 
> > Reported-by: Christian Borntraeger <borntraeger@...ibm.com>
> > Suggested-by: Christian Borntraeger <borntraeger@...ibm.com>
> > Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> > 
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index a4310a5..0e08d92 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -1970,11 +1970,22 @@ int hugepage_madvise(struct vm_area_struct *vma,
> >  {
> >  	switch (advice) {
> >  	case MADV_HUGEPAGE:
> > +#ifdef CONFIG_S390
> > +		/*
> > +		 * MADV_HUGEPAGE is broken after s390_enable_sie(), qemu
> > +		 * blindly does madvise(MADV_HUGEPAGE) for for all kvm pages
> > +		 * and expects it must fail on s390. Avoid a possible SIGSEGV
> > +		 * until qemu is changed.
> 
> I prefer:
> 		 * until kvm/s390 can handle large pages in the host.
> 
> Otherwise qemu has to be changed again, if we get THP working for kvm.
> 
> > +		 */
> > +		if (mm_has_pgste(vma->vm_mm))
> > +			return -EINVAL;
> > +#endif
> >  		/*
> >  		 * Be somewhat over-protective like KSM for now!
> >  		 */
> >  		if (*vm_flags & (VM_HUGEPAGE | VM_NO_THP))
> >  			return -EINVAL;
> > +
> 
> Unrelated white space?
> >  		*vm_flags &= ~VM_NOHUGEPAGE;
> >  		*vm_flags |= VM_HUGEPAGE;
> >  		/*
> > 
> 
> 
> 
> With the comment and white space change:
> 
> Tested-by: Christian Borntraeger <borntraeger@...ibm.com>
> Reviewed-by: Christian Borntraeger <borntraeger@...ibm.com>
> 
> Thanks for the quick patch

Yes, thank you all for the quick responses, and for looking over these
patches!
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