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

[Zi Yan <ziy@nvidia.com>]

On 19 Feb 2019, at 19:18, Mike Kravetz wrote:

> 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?

Yes, I assume the pages within the VMA are already populated but not 
contiguous yet.

My approach considers memory contiguity as an on-demand resource. In 
some phases
of an application, accelerators or RDMA controllers would 
process/transfer data in one
or more VMAs, at which time contiguous memory can help reduce address 
translation
overheads or lift certain constraints. And different VMAs could be 
processed at
different program phases, thus it might be hard to get contiguous memory 
for all
these VMAs at the allocation time using alloc_contig_pages(). My 
approach can
help get contiguous memory later, when the demand comes.

For some cases, you definitely can use alloc_contig_pages() to give 
users
a contiguous area at page allocation time, if you know the user is going 
to use this
area for accelerator data processing or as a RDMA buffer and the area 
size is fixed.

In addition, we can also use khugepaged approach, having a daemon 
periodically
scan VMAs and use alloc_contig_pages() to convert non-contiguous pages 
in a VMA
to contiguous pages, but it would require N free pages during the 
conversion.

In sum, my approach complements alloc_contig_pages() and provides more 
flexibility.
It is not trying to replaces alloc_contig_pages().


--
Best Regards,
Yan Zi