Re: [PATCHv4 08/25] thp: support file pages in zap_huge_pmd()

From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>,
	Matthew Wilcox <willy@linux.intel.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Hugh Dickins <hughd@google.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dave Hansen <dave.hansen@intel.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Christoph Lameter <cl@gentwo.org>,
	Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
	Jerome Marchand <jmarchan@redhat.com>,
	Yang Shi <yang.shi@linaro.org>,
	Sasha Levin <sasha.levin@oracle.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCHv4 08/25] thp: support file pages in zap_huge_pmd()
Date: Mon, 21 Mar 2016 22:12:00 +0530	[thread overview]
Message-ID: <87vb4fu6jr.fsf@linux.vnet.ibm.com> (raw)
In-Reply-To: <20160321143900.GA12917@node.shutemov.name>

"Kirill A. Shutemov" <kirill@shutemov.name> writes:

> [ text/plain ]
> On Mon, Mar 21, 2016 at 10:03:29AM +0530, Aneesh Kumar K.V wrote:
>> "Kirill A. Shutemov" <kirill@shutemov.name> writes:
>> 
>> > [ text/plain ]
>> > On Fri, Mar 18, 2016 at 07:23:41PM +0530, Aneesh Kumar K.V wrote:
>> >> "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> writes:
>> >> 
>> >> > [ text/plain ]
>> >> > split_huge_pmd() for file mappings (and DAX too) is implemented by just
>> >> > clearing pmd entry as we can re-fill this area from page cache on pte
>> >> > level later.
>> >> >
>> >> > This means we don't need deposit page tables when file THP is mapped.
>> >> > Therefore we shouldn't try to withdraw a page table on zap_huge_pmd()
>> >> > file THP PMD.
>> >> 
>> >> Archs like ppc64 use deposited page table to track the hardware page
>> >> table slot information. We probably may want to add hooks which arch can
>> >> use to achieve the same even with file THP 
>> >
>> > Could you describe more on what kind of information you're talking about?
>> >
>> 
>> Hardware page table in ppc64 requires us to map each subpage of the huge
>> page. This is needed because at low level we use segment base page size
>> to find the hash slot and on TLB miss, we use the faulting address and
>> base page size (which is 64k even with THP) to find whether we have
>> the page mapped in hash page table. Since we use base page size of 64K,
>> we need to make sure that subpages are mapped (on demand) in hash page
>> table. If we have then mapped we also need to track their hash table
>> slot information so that we can clear it on invalidate of hugepage.
>> 
>> With THP we used the deposited page table to store the hash slot
>> information.
>
> Could you prepare the patch with these hooks so we can discuss it details?
> I still have hard time wrap my had around this.

ok

>
> I think you have the same problem with DAX huge pages. Or you don't care
> about DAX?
>

Yes, DAX hugepage will not be working on ppc64. It is there in the TODO
list to get it working :).

-aneesh