linux-kernel - Re: [RFC PATCH 2/5] mm, arch: unify vmemmap

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Date:   Mon, 31 Jul 2017 16:27:46 +0200
From:   Gerald Schaefer <gerald.schaefer@...ibm.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Reza Arbab <arbab@...ux.vnet.ibm.com>,
        Yasuaki Ishimatsu <yasu.isimatu@...il.com>,
        qiuxishi@...wei.com, Kani Toshimitsu <toshi.kani@....com>,
        slaoub@...il.com, Joonsoo Kim <js1304@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tony Luck <tony.luck@...el.com>,
        Will Deacon <will.deacon@....com>, gerald.schaefer@...ibm.com
Subject: Re: [RFC PATCH 2/5] mm, arch: unify vmemmap_populate altmap
 handling

On Mon, 31 Jul 2017 14:55:56 +0200
Michal Hocko <mhocko@...nel.org> wrote:

> On Mon 31-07-17 14:40:53, Gerald Schaefer wrote:
> [...]
> > > @@ -247,12 +248,12 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node)
> > >  			 * use large frames even if they are only partially
> > >  			 * used.
> > >  			 * Otherwise we would have also page tables since
> > > -			 * vmemmap_populate gets called for each section
> > > +			 * __vmemmap_populate gets called for each section
> > >  			 * separately. */
> > >  			if (MACHINE_HAS_EDAT1) {
> > >  				void *new_page;
> > > 
> > > -				new_page = vmemmap_alloc_block(PMD_SIZE, node);
> > > +				new_page = __vmemmap_alloc_block_buf(PMD_SIZE, node, altmap);
> > >  				if (!new_page)
> > >  					goto out;
> > >  				pmd_val(*pm_dir) = __pa(new_page) | sgt_prot;
> > 
> > There is another call to vmemmap_alloc_block() in this function, a couple
> > of lines below, this should also be replaced by __vmemmap_alloc_block_buf().
> 
> I've noticed that one but in general I have only transformed PMD
> mappings because we shouldn't even get to pte level if the forme works
> AFAICS. Memory sections should be always 2MB aligned unless I am missing
> something. Or is this not true?

vmemmap_populate() on s390 will only stop at pmd level if we have HW
support for large pages (MACHINE_HAS_EDAT1). In that case we will allocate
a PMD_SIZE block with vmemmap_alloc_block() and map it on pmd level as
a large page.

Without HW large page support, we will continue to allocate a pte page,
populate the pmd entry with that, and fall through to the pte_none()
check below, with its PAGE_SIZE vmemmap_alloc_block() allocation. In this
case we should use the __vmemmap_alloc_block_buf().

Regards,
Gerald