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SubjectRe: [RFC PATCH 00/31] Generating physically contiguous memory after page allocation
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On 2/19/19 6:33 PM, Zi Yan wrote:
> 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.
>

Yes, that is how I expected the implementation would work. Thank you.

Another high level question. One of the benefits of this approach is
that exchanging pages does not require N free pages as you describe
above. This assumes that the vma which we are trying to make contiguous
is already populated. If it is not populated, then you also need to
have N free pages. Correct? If this is true, then is the expected use
case to first populate a vma, and then try to make contiguous? I would
assume that if it is not populated and a request to make contiguous is
given, we should try to allocate/populate the vma with contiguous pages
at that time?
--
Mike Kravetz

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