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From: Zi Yan <ziy@nvidia.com>
To: Mike Kravetz <mike.kravetz@oracle.com>
Cc: <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Michal Hocko <mhocko@kernel.org>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	Mel Gorman <mgorman@techsingularity.net>,
	John Hubbard <jhubbard@nvidia.com>,
	Mark Hairgrove <mhairgrove@nvidia.com>,
	Nitin Gupta <nigupta@nvidia.com>,
	David Nellans <dnellans@nvidia.com>
Subject: Re: [RFC PATCH 00/31] Generating physically contiguous memory after page allocation
Date: Tue, 19 Feb 2019 18:33:35 -0800
Message-ID: <FDDDB4C8-C5B5-46B0-9682-33AC063F7A46@nvidia.com> (raw)
In-Reply-To: <f4cf53a3-359b-8c66-ed15-112b3cf0f475@oracle.com>

On 19 Feb 2019, at 17:42, Mike Kravetz wrote:

> On 2/15/19 2:08 PM, Zi Yan wrote:
>
> Thanks for working on this issue!
>
> I have not yet had a chance to take a look at the code.  However, I do 
> have
> some general questions/comments on the approach.

Thanks for replying. The code is very intrusive and has a lot of hacks, 
so it is
OK for us to discuss the general idea first. :)


>> Patch structure
>> ----
>>
>> The patchset I developed to generate physically contiguous 
>> memory/arbitrary
>> sized pages merely moves pages around. There are three components in 
>> this
>> patchset:
>>
>> 1) a new page migration mechanism, called exchange pages, that 
>> exchanges the
>> content of two in-use pages instead of performing two back-to-back 
>> page
>> migration. It saves on overheads and avoids page reclaim and memory 
>> compaction
>> in the page allocation path, although it is not strictly required if 
>> enough
>> free memory is available in the system.
>>
>> 2) a new mechanism that utilizes both page migration and exchange 
>> pages to
>> produce physically contiguous memory/arbitrary sized pages without 
>> allocating
>> any new pages, unlike what khugepaged does. It works on per-VMA 
>> basis, creating
>> physically contiguous memory out of each VMA, which is virtually 
>> contiguous.
>> A simple range tree is used to ensure no two VMAs are overlapping 
>> with each
>> other in the physical address space.
>
> This appears to be a new approach to generating contiguous areas.  
> Previous
> attempts had relied on finding a contiguous area that can then be used 
> for
> various purposes including user mappings.  Here, you take an existing 
> mapping
> and make it contiguous.  [RFC PATCH 04/31] mm: add mem_defrag 
> functionality
> talks about creating a (VPN, PFN) anchor pair for each vma and then 
> using
> this pair as the base for creating a contiguous area.
>
> I'm curious, how 'fixed' is the anchor?  As you know, there could be a
> non-movable page in the PFN range.  As a result, you will not be able 
> to
> create a contiguous area starting at that PFN.  In such a case, do we 
> try
> another PFN?  I know this could result in much page shuffling.  I'm 
> just
> trying to figure out how we satisfy a user who really wants a 
> contiguous
> area.  Is there some method to keep trying?

Good question. The anchor is determined on a per-VMA basis, which can be 
changed easily,
but in this patchiest, I used a very simple strategy — making all VMAs 
not overlapping
in the physical address space to get maximum overall contiguity and not 
changing anchors
even if non-moveable pages are encountered when generating physically 
contiguous pages.

Basically, first VMA1 in the virtual address space has its anchor as 
(VMA1_start_VPN, ZONE_start_PFN),
second VMA1 has its anchor as (VMA2_start_VPN, ZONE_start_PFN + 
VMA1_size), and so on.
This makes all VMA not overlapping in physical address space during 
contiguous memory
generation. When there is a non-moveable page, the anchor will not be 
changed, because
no matter whether we assign a new anchor or not, the contiguous pages 
stops at
the non-moveable page. If we are trying to get a new anchor, more effort 
is needed to
avoid overlapping new anchor with existing contiguous pages. Any 
overlapping will
nullify the existing contiguous pages.

To satisfy a user who wants a contiguous area with N pages, the minimal 
distance between
any two non-moveable pages should be bigger than N pages in the system 
memory. Otherwise,
nothing would work. If there is such an area (PFN1, PFN1+N) in the 
physical address space,
you can set the anchor to (VPN_USER, PFN1) and use exchange_pages() to 
generate a contiguous
area with N pages. Instead, alloc_contig_pages(PFN1, PFN1+N, …) could 
also work, but
only at page allocation time. It also requires the system has N free 
pages when
alloc_contig_pages() are migrating the pages in (PFN1, PFN1+N) away, or 
you need to swap
pages to make the space.

Let me know if this makes sense to you.

--
Best Regards,
Yan Zi


  reply index

Thread overview: 50+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-15 22:08 Zi Yan
2019-02-15 22:08 ` [RFC PATCH 01/31] mm: migrate: Add exchange_pages to exchange two lists of pages Zi Yan
2019-02-17 11:29   ` Matthew Wilcox
2019-02-18 17:31     ` Zi Yan
2019-02-18 17:42       ` Vlastimil Babka
2019-02-18 17:51         ` Zi Yan
2019-02-18 17:52           ` Matthew Wilcox
2019-02-18 17:59             ` Zi Yan
2019-02-19  7:42               ` Anshuman Khandual
2019-02-19 12:56                 ` Matthew Wilcox
2019-02-20  4:38                   ` Anshuman Khandual
2019-03-14  2:39                     ` Zi Yan
2019-02-21 21:10   ` Jerome Glisse
2019-02-21 21:25     ` Zi Yan
2019-02-15 22:08 ` [RFC PATCH 02/31] mm: migrate: Add THP exchange support Zi Yan
2019-02-15 22:08 ` [RFC PATCH 03/31] mm: migrate: Add tmpfs " Zi Yan
2019-02-15 22:08 ` [RFC PATCH 04/31] mm: add mem_defrag functionality Zi Yan
2019-02-15 22:08 ` [RFC PATCH 05/31] mem_defrag: split a THP if either src or dst is THP only Zi Yan
2019-02-15 22:08 ` [RFC PATCH 06/31] mm: Make MAX_ORDER configurable in Kconfig for buddy allocator Zi Yan
2019-02-15 22:08 ` [RFC PATCH 07/31] mm: deallocate pages with order > MAX_ORDER Zi Yan
2019-02-15 22:08 ` [RFC PATCH 08/31] mm: add pagechain container for storing multiple pages Zi Yan
2019-02-15 22:08 ` [RFC PATCH 09/31] mm: thp: 1GB anonymous page implementation Zi Yan
2019-02-15 22:08 ` [RFC PATCH 10/31] mm: proc: add 1GB THP kpageflag Zi Yan
2019-02-15 22:08 ` [RFC PATCH 11/31] mm: debug: print compound page order in dump_page() Zi Yan
2019-02-15 22:08 ` [RFC PATCH 12/31] mm: stats: Separate PMD THP and PUD THP stats Zi Yan
2019-02-15 22:08 ` [RFC PATCH 13/31] mm: thp: 1GB THP copy on write implementation Zi Yan
2019-02-15 22:08 ` [RFC PATCH 14/31] mm: thp: handling 1GB THP reference bit Zi Yan
2019-02-15 22:08 ` [RFC PATCH 15/31] mm: thp: add 1GB THP split_huge_pud_page() function Zi Yan
2019-02-15 22:08 ` [RFC PATCH 16/31] mm: thp: check compound_mapcount of PMD-mapped PUD THPs at free time Zi Yan
2019-02-15 22:08 ` [RFC PATCH 17/31] mm: thp: split properly PMD-mapped PUD THP to PTE-mapped PUD THP Zi Yan
2019-02-15 22:08 ` [RFC PATCH 18/31] mm: page_vma_walk: teach it about PMD-mapped " Zi Yan
2019-02-15 22:08 ` [RFC PATCH 19/31] mm: thp: 1GB THP support in try_to_unmap() Zi Yan
2019-02-15 22:08 ` [RFC PATCH 20/31] mm: thp: split 1GB THPs at page reclaim Zi Yan
2019-02-15 22:08 ` [RFC PATCH 21/31] mm: thp: 1GB zero page shrinker Zi Yan
2019-02-15 22:08 ` [RFC PATCH 22/31] mm: thp: 1GB THP follow_p*d_page() support Zi Yan
2019-02-15 22:08 ` [RFC PATCH 23/31] mm: support 1GB THP pagemap support Zi Yan
2019-02-15 22:08 ` [RFC PATCH 24/31] sysctl: add an option to only print the head page virtual address Zi Yan
2019-02-15 22:08 ` [RFC PATCH 25/31] mm: thp: add a knob to enable/disable 1GB THPs Zi Yan
2019-02-15 22:08 ` [RFC PATCH 26/31] mm: thp: promote PTE-mapped THP to PMD-mapped THP Zi Yan
2019-02-15 22:08 ` [RFC PATCH 27/31] mm: thp: promote PMD-mapped PUD pages to PUD-mapped PUD pages Zi Yan
2019-02-15 22:08 ` [RFC PATCH 28/31] mm: vmstats: add page promotion stats Zi Yan
2019-02-15 22:08 ` [RFC PATCH 29/31] mm: madvise: add madvise options to split PMD and PUD THPs Zi Yan
2019-02-15 22:08 ` [RFC PATCH 30/31] mm: mem_defrag: thp: PMD THP and PUD THP in-place promotion support Zi Yan
2019-02-15 22:08 ` [RFC PATCH 31/31] sysctl: toggle to promote PUD-mapped 1GB THP or not Zi Yan
2019-02-20  1:42 ` [RFC PATCH 00/31] Generating physically contiguous memory after page allocation Mike Kravetz
2019-02-20  2:33   ` Zi Yan [this message]
2019-02-20  3:18     ` Mike Kravetz
2019-02-20  5:19       ` Zi Yan
2019-02-20  5:27         ` Mike Kravetz
  -- strict thread matches above, loose matches on Subject: below --
2019-02-15 22:03 Zi Yan

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