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

[Mike Kravetz]

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.

> 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?

My apologies if this is addressed in the code.  This was just one of the
first thoughts that came to mine when giving the series a quick look.
-- 
Mike Kravetz