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From: Joao Martins <joao.m.martins@oracle.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: linux-mm@kvack.org, linux-nvdimm@lists.01.org,
	Muchun Song <songmuchun@bytedance.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jason Gunthorpe <jgg@ziepe.ca>
Subject: Re: [PATCH RFC 6/9] mm/gup: Grab head page refcount once for group of subpages
Date: Thu, 10 Dec 2020 15:43:22 +0000	[thread overview]
Message-ID: <996c74e8-fa86-bb91-6e95-b4b7efac7c05@oracle.com> (raw)
In-Reply-To: <20201209181406.GQ7338@casper.infradead.org>



On 12/9/20 6:14 PM, Matthew Wilcox wrote:
> On Wed, Dec 09, 2020 at 12:24:38PM -0400, Jason Gunthorpe wrote:
>> On Wed, Dec 09, 2020 at 04:02:05PM +0000, Joao Martins wrote:
>>
>>> Today (without the series) struct pages are not represented the way they
>>> are expressed in the page tables, which is what I am hoping to fix in this
>>> series thus initializing these as compound pages of a given order. But me
>>> introducing PGMAP_COMPOUND was to conservatively keep both old (non-compound)
>>> and new (compound pages) co-exist.
>>
>> Oooh, that I didn't know.. That is kind of horrible to have a PMD
>> pointing at an order 0 page only in this one special case.
> 
> Uh, yes.  I'm surprised it hasn't caused more problems.
> 
There was 1 or 2 problems in the KVM MMU related to zone device pages.

See commit e851265a816f ("KVM: x86/mmu: Use huge pages for DAX-backed files")
which eventually lead to commit db5432165e9b5 ("KVM: x86/mmu: Walk host page
tables to find THP mappings") to be less amenable to metadata changes.

>> Still, I think it would be easier to teach record_subpages() that a
>> PMD doesn't necessarily point to a high order page, eg do something
>> like I suggested for the SGL where it extracts the page order and
>> iterates over the contiguous range of pfns.
> 
> But we also see good performance improvements from doing all reference
> counts on the head page instead of spread throughout the pages, so we
> really want compound pages.

Going further than just refcounts and borrowing your (or someone else?)
idea, perhaps also a FOLL_HEAD gup flag that would let us only work with
head pages (or folios). Which would consequently let us pin/grab bigger
swathes of memory e.g. 1G (in 2M head pages) or 512G (in 1G head pages)
with just 1 page for storing the struct pages[*]. Albeit I suspect the
numbers would have to justify it.

	Joao

[*] One page happens to be what's used for RDMA/umem and vdpa as callers
of pin_user_pages*()


  parent reply	other threads:[~2020-12-10 15:43 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
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 [this message]
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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