From: Dan Williams <firstname.lastname@example.org> To: Joao Martins <email@example.com> Cc: Linux MM <firstname.lastname@example.org>, Ira Weiny <email@example.com>, linux-nvdimm <firstname.lastname@example.org>, Matthew Wilcox <email@example.com>, Jason Gunthorpe <firstname.lastname@example.org>, Jane Chu <email@example.com>, Muchun Song <firstname.lastname@example.org>, Mike Kravetz <email@example.com>, Andrew Morton <firstname.lastname@example.org>, John Hubbard <email@example.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: <firstname.lastname@example.org> On Wed, Mar 10, 2021 at 10:13 AM Joao Martins <email@example.com> wrote: > > On 2/22/21 8:37 PM, Dan Williams wrote: > > On Mon, Feb 22, 2021 at 3:24 AM Joao Martins <firstname.lastname@example.org> wrote: > >> On 2/20/21 1:43 AM, Dan Williams wrote: > >>> On Tue, Dec 8, 2020 at 9:59 PM John Hubbard <email@example.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.
next prev parent reply other threads:[~2021-03-12 5:54 UTC|newest] Thread overview: 80+ 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 2020-12-08 19:49 ` Jason Gunthorpe 2020-12-09 11:05 ` Joao Martins 2020-12-09 15:15 ` Jason Gunthorpe 2020-12-09 16:02 ` Joao Martins 2020-12-09 16:24 ` Jason Gunthorpe 2020-12-09 17:27 ` Joao Martins 2020-12-09 18:14 ` Matthew Wilcox 2020-12-09 19:08 ` Jason Gunthorpe 2020-12-10 15:43 ` Joao Martins 2020-12-09 4:40 ` John Hubbard 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 19:34 ` Jason Gunthorpe 2020-12-09 5:06 ` John Hubbard 2020-12-09 13:43 ` Jason Gunthorpe 2020-12-09 12:17 ` Joao Martins 2020-12-17 19:05 ` Joao Martins 2020-12-17 20:05 ` Jason Gunthorpe 2020-12-17 22:34 ` Joao Martins 2020-12-18 14:25 ` Jason Gunthorpe 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 2020-12-08 19:29 ` Jason Gunthorpe 2020-12-09 10:59 ` Joao Martins 2020-12-19 13:15 ` Joao Martins 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 19:57 ` Jason Gunthorpe 2020-12-09 8:05 ` Christoph Hellwig 2020-12-09 11:19 ` Joao Martins 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 9:52 ` [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 2021-02-23 18:54 ` Jason Gunthorpe 2021-02-23 22:48 ` Dan Williams 2021-02-23 23:07 ` Jason Gunthorpe 2021-02-24 0:14 ` Dan Williams 2021-02-24 1:00 ` Jason Gunthorpe 2021-02-24 1:32 ` Dan Williams
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