From: Dan Williams <dan.j.williams@intel.com>
To: Joao Martins <joao.m.martins@oracle.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: Thu, 11 Mar 2021 21:54:40 -0800 [thread overview]
Message-ID: <CAPcyv4gAfBO18bPvc=yRbAhaOuDcDgz2wOsvs+konvfpxDiJeA@mail.gmail.com> (raw)
In-Reply-To: <b1ef0aa7-754c-78e8-ffbd-87397c1eaf79@oracle.com>
On Wed, Mar 10, 2021 at 10:13 AM Joao Martins <joao.m.martins@oracle.com> wrote:
>
> 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?
The space savings is the whole point. So I agree with moving altmap
support to a follow-on enhancement, but land the non-altmap basepage
support in the first round.
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next prev parent reply other threads:[~2021-03-12 5:54 UTC|newest]
Thread overview: 67+ 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 ` [PATCH RFC 1/9] memremap: add ZONE_DEVICE support for compound pages Joao Martins
2020-12-09 5:59 ` John Hubbard
2020-12-09 6:33 ` Matthew Wilcox
2020-12-09 13:12 ` Joao Martins
2021-02-20 1:43 ` Dan Williams
2021-02-22 11:24 ` Joao Martins
2021-02-22 20:37 ` Dan Williams
2021-02-23 15:46 ` Joao Martins
2021-02-23 16:50 ` Dan Williams
2021-02-23 17:18 ` Joao Martins
2021-02-23 18:18 ` Dan Williams
2021-03-10 18:12 ` Joao Martins
2021-03-12 5:54 ` Dan Williams [this message]
2021-02-20 1:24 ` Dan Williams
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
2020-12-09 6:16 ` John Hubbard
2020-12-09 13:51 ` Joao Martins
2021-02-20 1:49 ` Dan Williams
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 17:38 ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 3/9] sparse-vmemmap: Reuse vmemmap areas for a given page size Joao Martins
2021-02-20 3:34 ` Dan Williams
2021-02-22 11:42 ` Joao Martins
2021-02-22 22:40 ` Dan Williams
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
2021-02-20 6:17 ` Dan Williams
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 ` [PATCH RFC 6/9] mm/gup: Grab head page refcount once for group of subpages Joao Martins
2020-12-09 4:40 ` John Hubbard
2020-12-09 13:44 ` Joao Martins
[not found] ` <20201208194905.GQ5487@ziepe.ca>
2020-12-09 11:05 ` Joao Martins
[not found] ` <20201209151505.GV5487@ziepe.ca>
2020-12-09 16:02 ` Joao Martins
[not found] ` <20201209162438.GW5487@ziepe.ca>
2020-12-09 17:27 ` Joao Martins
2020-12-09 18:14 ` Matthew Wilcox
2020-12-10 15:43 ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 7/9] mm/gup: Decrement head page " Joao Martins
[not found] ` <20201208193446.GP5487@ziepe.ca>
2020-12-09 5:06 ` John Hubbard
2020-12-09 12:17 ` Joao Martins
2020-12-17 19:05 ` Joao Martins
[not found] ` <20201217200530.GK5487@ziepe.ca>
2020-12-17 22:34 ` Joao Martins
2020-12-19 2:06 ` John Hubbard
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
2020-12-09 5:18 ` John Hubbard
[not found] ` <20201208192935.GA1908088@ziepe.ca>
2020-12-09 10:59 ` Joao Martins
2020-12-19 13:15 ` Joao Martins
2020-12-08 17:29 ` [PATCH RFC 9/9] mm: Add follow_devmap_page() for devdax vmas Joao Martins
2020-12-09 5:23 ` John Hubbard
[not found] ` <20201208195754.GR5487@ziepe.ca>
2020-12-09 8:05 ` Christoph Hellwig
2020-12-09 11:19 ` Joao Martins
2020-12-09 9:38 ` [PATCH RFC 0/9] mm, sparse-vmemmap: Introduce compound pagemaps David Hildenbrand
2020-12-09 9:52 ` [External] " Muchun Song
2021-02-20 1:18 ` Dan Williams
2021-02-22 11:06 ` Joao Martins
2021-02-22 14:32 ` Joao Martins
2021-02-23 16:28 ` Joao Martins
2021-02-23 16:44 ` Dan Williams
2021-02-23 17:15 ` Joao Martins
2021-02-23 18:15 ` Dan Williams
[not found] ` <20210223185435.GO2643399@ziepe.ca>
2021-02-23 22:48 ` Dan Williams
[not found] ` <20210223230723.GP2643399@ziepe.ca>
2021-02-24 0:14 ` Dan Williams
[not found] ` <20210224010017.GQ2643399@ziepe.ca>
2021-02-24 1:32 ` Dan Williams
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