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

[Zi Yan]

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 â 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, â) 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