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From: Joao Martins <joao.m.martins@oracle.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Linux MM <linux-mm@kvack.org>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	Matthew Wilcox <willy@infradead.org>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	Muchun Song <songmuchun@bytedance.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	John Hubbard <jhubbard@nvidia.com>
Subject: Re: [PATCH RFC 1/9] memremap: add ZONE_DEVICE support for compound pages
Date: Wed, 10 Mar 2021 18:12:47 +0000	[thread overview]
Message-ID: <b1ef0aa7-754c-78e8-ffbd-87397c1eaf79@oracle.com> (raw)
In-Reply-To: <CAPcyv4jDk=ppsR2Pvgpb1DqWk5D8bkrNCAtyRU21ShnC3fzdSA@mail.gmail.com>

On 2/22/21 8:37 PM, Dan Williams wrote:
> On Mon, Feb 22, 2021 at 3:24 AM Joao Martins <joao.m.martins@oracle.com> wrote:
>> On 2/20/21 1:43 AM, Dan Williams wrote:
>>> On Tue, Dec 8, 2020 at 9:59 PM John Hubbard <jhubbard@nvidia.com> wrote:
>>>> On 12/8/20 9:28 AM, Joao Martins wrote:
>> One thing to point out about altmap is that the degradation (in pinning and
>> unpining) we observed with struct page's in device memory, is no longer observed
>> once 1) we batch ref count updates as we move to compound pages 2) reusing
>> tail pages seems to lead to these struct pages staying more likely in cache
>> which perhaps contributes to dirtying a lot less cachelines.
> 
> True, it makes it more palatable to survive 'struct page' in PMEM, 

I want to retract for now what I said above wrt to the no degradation with
struct page in device comment. I was fooled by a bug on a patch later down
this series. Particular because I accidentally cleared PGMAP_ALTMAP_VALID when
unilaterally setting PGMAP_COMPOUND, which consequently lead to always
allocating struct pages from memory. No wonder the numbers were just as fast.
I am still confident that it's going to be faster and observe less degradation
in pinning/init. Init for now is worst-case 2x faster. But to be *as fast* struct
pages in memory might still be early to say.

The broken masking of the PGMAP_ALTMAP_VALID bit did hide one flaw, where
we don't support altmap for basepages on x86/mm and it apparently depends
on architectures to implement it (and a couple other issues). The vmemmap
allocation isn't the problem, so the previous comment in this thread that
altmap doesn't change much in the vmemmap_populate_compound_pages() is
still accurate.

The problem though resides on the freeing of vmemmap pagetables with
basepages *with altmap* (e.g. at dax-device teardown) which require arch
support. Doing it properly would mean making the altmap reserve smaller
(given fewer pages are allocated), and the ability for the altmap pfn
allocator to track references per pfn. But I think it deserves its own
separate patch series (probably almost just as big).

Perhaps for this set I can stick without altmap as you suggested, and
use hugepage vmemmap population (which wouldn't
lead to device memory savings) instead of reusing base pages . I would
still leave the compound page support logic as metadata representation
for > 4K @align, as I think that's the right thing to do. And then
a separate series onto improving altmap to leverage the metadata reduction
support as done with non-device struct pages.

Thoughts?

	Joao
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WARNING: multiple messages have this Message-ID
From: Joao Martins <joao.m.martins@oracle.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Linux MM <linux-mm@kvack.org>, Ira Weiny <ira.weiny@intel.com>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	Matthew Wilcox <willy@infradead.org>,
	Jason Gunthorpe <jgg@ziepe.ca>, Jane Chu <jane.chu@oracle.com>,
	Muchun Song <songmuchun@bytedance.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	John Hubbard <jhubbard@nvidia.com>
Subject: Re: [PATCH RFC 1/9] memremap: add ZONE_DEVICE support for compound pages
Date: Wed, 10 Mar 2021 18:12:47 +0000	[thread overview]
Message-ID: <b1ef0aa7-754c-78e8-ffbd-87397c1eaf79@oracle.com> (raw)
In-Reply-To: <CAPcyv4jDk=ppsR2Pvgpb1DqWk5D8bkrNCAtyRU21ShnC3fzdSA@mail.gmail.com>

On 2/22/21 8:37 PM, Dan Williams wrote:
> On Mon, Feb 22, 2021 at 3:24 AM Joao Martins <joao.m.martins@oracle.com> wrote:
>> On 2/20/21 1:43 AM, Dan Williams wrote:
>>> On Tue, Dec 8, 2020 at 9:59 PM John Hubbard <jhubbard@nvidia.com> wrote:
>>>> On 12/8/20 9:28 AM, Joao Martins wrote:
>> One thing to point out about altmap is that the degradation (in pinning and
>> unpining) we observed with struct page's in device memory, is no longer observed
>> once 1) we batch ref count updates as we move to compound pages 2) reusing
>> tail pages seems to lead to these struct pages staying more likely in cache
>> which perhaps contributes to dirtying a lot less cachelines.
> 
> True, it makes it more palatable to survive 'struct page' in PMEM, 

I want to retract for now what I said above wrt to the no degradation with
struct page in device comment. I was fooled by a bug on a patch later down
this series. Particular because I accidentally cleared PGMAP_ALTMAP_VALID when
unilaterally setting PGMAP_COMPOUND, which consequently lead to always
allocating struct pages from memory. No wonder the numbers were just as fast.
I am still confident that it's going to be faster and observe less degradation
in pinning/init. Init for now is worst-case 2x faster. But to be *as fast* struct
pages in memory might still be early to say.

The broken masking of the PGMAP_ALTMAP_VALID bit did hide one flaw, where
we don't support altmap for basepages on x86/mm and it apparently depends
on architectures to implement it (and a couple other issues). The vmemmap
allocation isn't the problem, so the previous comment in this thread that
altmap doesn't change much in the vmemmap_populate_compound_pages() is
still accurate.

The problem though resides on the freeing of vmemmap pagetables with
basepages *with altmap* (e.g. at dax-device teardown) which require arch
support. Doing it properly would mean making the altmap reserve smaller
(given fewer pages are allocated), and the ability for the altmap pfn
allocator to track references per pfn. But I think it deserves its own
separate patch series (probably almost just as big).

Perhaps for this set I can stick without altmap as you suggested, and
use hugepage vmemmap population (which wouldn't
lead to device memory savings) instead of reusing base pages . I would
still leave the compound page support logic as metadata representation
for > 4K @align, as I think that's the right thing to do. And then
a separate series onto improving altmap to leverage the metadata reduction
support as done with non-device struct pages.

Thoughts?

	Joao


  parent reply	other threads:[~2021-03-10 18:13 UTC|newest]

Thread overview: 147+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-08 17:28 [PATCH RFC 0/9] mm, sparse-vmemmap: Introduce compound pagemaps Joao Martins
2020-12-08 17:28 ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 1/9] memremap: add ZONE_DEVICE support for compound pages Joao Martins
2020-12-08 17:28   ` Joao Martins
2020-12-09  5:59   ` John Hubbard
2020-12-09  5:59     ` John Hubbard
2020-12-09  6:33     ` Matthew Wilcox
2020-12-09  6:33       ` Matthew Wilcox
2020-12-09 13:12       ` Joao Martins
2020-12-09 13:12         ` Joao Martins
2021-02-20  1:43     ` Dan Williams
2021-02-20  1:43       ` Dan Williams
2021-02-22 11:24       ` Joao Martins
2021-02-22 11:24         ` Joao Martins
2021-02-22 20:37         ` Dan Williams
2021-02-22 20:37           ` Dan Williams
2021-02-23 15:46           ` Joao Martins
2021-02-23 15:46             ` Joao Martins
2021-02-23 16:50             ` Dan Williams
2021-02-23 16:50               ` Dan Williams
2021-02-23 17:18               ` Joao Martins
2021-02-23 17:18                 ` Joao Martins
2021-02-23 18:18                 ` Dan Williams
2021-02-23 18:18                   ` Dan Williams
2021-03-10 18:12           ` Joao Martins [this message]
2021-03-10 18:12             ` Joao Martins
2021-03-12  5:54             ` Dan Williams
2021-03-12  5:54               ` Dan Williams
2021-02-20  1:24   ` Dan Williams
2021-02-20  1:24     ` Dan Williams
2021-02-22 11:09     ` Joao Martins
2021-02-22 11:09       ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 2/9] sparse-vmemmap: Consolidate arguments in vmemmap section populate Joao Martins
2020-12-08 17:28   ` Joao Martins
2020-12-09  6:16   ` John Hubbard
2020-12-09  6:16     ` John Hubbard
2020-12-09 13:51     ` Joao Martins
2020-12-09 13:51       ` Joao Martins
2021-02-20  1:49   ` Dan Williams
2021-02-20  1:49     ` Dan Williams
2021-02-22 11:26     ` Joao Martins
2021-02-22 11:26       ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 3/9] sparse-vmemmap: Reuse vmemmap areas for a given mhp_params::align Joao Martins
2020-12-08 17:28   ` Joao Martins
2020-12-08 17:38   ` Joao Martins
2020-12-08 17:38     ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 3/9] sparse-vmemmap: Reuse vmemmap areas for a given page size Joao Martins
2020-12-08 17:28   ` Joao Martins
2021-02-20  3:34   ` Dan Williams
2021-02-20  3:34     ` Dan Williams
2021-02-22 11:42     ` Joao Martins
2021-02-22 11:42       ` Joao Martins
2021-02-22 22:40       ` Dan Williams
2021-02-22 22:40         ` Dan Williams
2021-02-23 15:46         ` Joao Martins
2021-02-23 15:46           ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 4/9] mm/page_alloc: Reuse tail struct pages for compound pagemaps Joao Martins
2020-12-08 17:28   ` Joao Martins
2021-02-20  6:17   ` Dan Williams
2021-02-20  6:17     ` Dan Williams
2021-02-22 12:01     ` Joao Martins
2021-02-22 12:01       ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 5/9] device-dax: Compound pagemap support Joao Martins
2020-12-08 17:28   ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 6/9] mm/gup: Grab head page refcount once for group of subpages Joao Martins
2020-12-08 17:28   ` Joao Martins
2020-12-08 19:49   ` Jason Gunthorpe
2020-12-09 11:05     ` Joao Martins
2020-12-09 11:05       ` Joao Martins
2020-12-09 15:15       ` Jason Gunthorpe
2020-12-09 16:02         ` Joao Martins
2020-12-09 16:02           ` Joao Martins
2020-12-09 16:24           ` Jason Gunthorpe
2020-12-09 17:27             ` Joao Martins
2020-12-09 17:27               ` Joao Martins
2020-12-09 18:14             ` Matthew Wilcox
2020-12-09 18:14               ` Matthew Wilcox
2020-12-09 19:08               ` Jason Gunthorpe
2020-12-10 15:43               ` Joao Martins
2020-12-10 15:43                 ` Joao Martins
2020-12-09  4:40   ` John Hubbard
2020-12-09  4:40     ` John Hubbard
2020-12-09 13:44     ` Joao Martins
2020-12-09 13:44       ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 7/9] mm/gup: Decrement head page " Joao Martins
2020-12-08 17:28   ` Joao Martins
2020-12-08 19:34   ` Jason Gunthorpe
2020-12-09  5:06     ` John Hubbard
2020-12-09  5:06       ` John Hubbard
2020-12-09 13:43       ` Jason Gunthorpe
2020-12-09 12:17     ` Joao Martins
2020-12-09 12:17       ` Joao Martins
2020-12-17 19:05     ` Joao Martins
2020-12-17 19:05       ` Joao Martins
2020-12-17 20:05       ` Jason Gunthorpe
2020-12-17 22:34         ` Joao Martins
2020-12-17 22:34           ` Joao Martins
2020-12-18 14:25           ` Jason Gunthorpe
2020-12-19  2:06         ` John Hubbard
2020-12-19  2:06           ` John Hubbard
2020-12-19 13:10           ` Joao Martins
2020-12-19 13:10             ` Joao Martins
2020-12-08 17:29 ` [PATCH RFC 8/9] RDMA/umem: batch page unpin in __ib_mem_release() Joao Martins
2020-12-08 17:29   ` Joao Martins
2020-12-08 19:29   ` Jason Gunthorpe
2020-12-09 10:59     ` Joao Martins
2020-12-09 10:59       ` Joao Martins
2020-12-19 13:15       ` Joao Martins
2020-12-19 13:15         ` Joao Martins
2020-12-09  5:18   ` John Hubbard
2020-12-09  5:18     ` John Hubbard
2020-12-08 17:29 ` [PATCH RFC 9/9] mm: Add follow_devmap_page() for devdax vmas Joao Martins
2020-12-08 17:29   ` Joao Martins
2020-12-08 19:57   ` Jason Gunthorpe
2020-12-09  8:05     ` Christoph Hellwig
2020-12-09  8:05       ` Christoph Hellwig
2020-12-09 11:19     ` Joao Martins
2020-12-09 11:19       ` Joao Martins
2020-12-09  5:23   ` John Hubbard
2020-12-09  5:23     ` John Hubbard
2020-12-09  9:38 ` [PATCH RFC 0/9] mm, sparse-vmemmap: Introduce compound pagemaps David Hildenbrand
2020-12-09  9:38   ` David Hildenbrand
2020-12-09  9:52 ` [External] " Muchun Song
2020-12-09  9:52   ` Muchun Song
2021-02-20  1:18 ` Dan Williams
2021-02-20  1:18   ` Dan Williams
2021-02-22 11:06   ` Joao Martins
2021-02-22 11:06     ` Joao Martins
2021-02-22 14:32     ` Joao Martins
2021-02-22 14:32       ` Joao Martins
2021-02-23 16:28   ` Joao Martins
2021-02-23 16:28     ` Joao Martins
2021-02-23 16:44     ` Dan Williams
2021-02-23 16:44       ` Dan Williams
2021-02-23 17:15       ` Joao Martins
2021-02-23 17:15         ` Joao Martins
2021-02-23 18:15         ` Dan Williams
2021-02-23 18:15           ` Dan Williams
2021-02-23 18:54       ` Jason Gunthorpe
2021-02-23 22:48         ` Dan Williams
2021-02-23 22:48           ` Dan Williams
2021-02-23 23:07           ` Jason Gunthorpe
2021-02-24  0:14             ` Dan Williams
2021-02-24  0:14               ` Dan Williams
2021-02-24  1:00               ` Jason Gunthorpe
2021-02-24  1:32                 ` Dan Williams
2021-02-24  1:32                   ` Dan Williams

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