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SubjectRe: [RFC PATCH 00/31] Generating physically contiguous memory after page allocation
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

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