Re: [RFC PATCH kernel vfio] mm: vfio: Move pages out of CMA before pinning

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 08/17/2015 05:45 PM, Vlastimil Babka wrote:
> On 08/05/2015 10:08 AM, Alexey Kardashevskiy wrote:
>> This is about VFIO aka PCI passthrough used from QEMU.
>> KVM is irrelevant here.
>>
>> QEMU is a machine emulator. It allocates guest RAM from anonymous memory
>> and these pages are movable which is ok. They may happen to be allocated
>> from the contiguous memory allocation zone (CMA). Which is also ok as
>> long they are movable.
>>
>> However if the guest starts using VFIO (which can be hotplugged into
>> the guest), in most cases it involves DMA which requires guest RAM pages
>> to be pinned and not move once their addresses are programmed to
>> the hardware for DMA.
>>
>> So we end up in a situation when quite many pages in CMA are not movable
>> anymore. And we get bunch of these:
>>
>> [77306.513966] alloc_contig_range: [1f3800, 1f78c4) PFNs busy
>> [77306.514448] alloc_contig_range: [1f3800, 1f78c8) PFNs busy
>> [77306.514927] alloc_contig_range: [1f3800, 1f78cc) PFNs busy
>
> IIRC CMA was for mobile devices and their camera/codec drivers and you
> don't use QEMU on those? What do you need CMA for in your case?


I do not want QEMU to get memory from CMA, this is my point. It just 
happens sometime that the kernel allocates movable pages from there.


>
>> This is a very rough patch to start the conversation about how to move
>> pages properly. mm/page_alloc.c does this and
>> arch/powerpc/mm/mmu_context_iommu.c exploits it.
>
> OK such conversation should probably start by mentioning the VM_PINNED
> effort by Peter Zijlstra: https://lkml.org/lkml/2014/5/26/345
>
> It's more general approach to dealing with pinned pages, and moving them
> out of CMA area (and compacting them in general) prior pinning is one of
> the things that should be done within that framework.


And I assume these patches did not go anywhere, right?...



> Then there's the effort to enable migrating pages other than LRU during
> compaction (and thus CMA allocation): https://lwn.net/Articles/650864/
> I don't know if that would be applicable in your use case, i.e. are the
> pins for DMA short-lived and can the isolation/migration code wait a bit
> for the transfer to finish so it can grab them, or something?


Pins for DMA are long-lived, pretty much as long as the guest is running. 
So this "compaction" is too late.


>>
>> Please do not comment on the style and code placement,
>> this is just to give some context :)
>>
>> Obviously, this does not work well - it manages to migrate only few pages
>> and crashes as it is missing locks/disabling interrupts and I probably
>> should not just remove pages from LRU list (normally, I guess, only these
>> can migrate) and a million of other things.
>>
>> The questions are:
>>
>> - what is the correct way of telling if the page is in CMA?
>> is (get_pageblock_migratetype(page) == MIGRATE_CMA) good enough?
>
> Should be.
>
>> - how to tell MM to move page away? I am calling migrate_pages() with
>> an get_new_page callback which allocates a page with GFP_USER but without
>> GFP_MOVABLE which should allocate new page out of CMA which seems ok but
>> there is a little convern that we might want to add MOVABLE back when
>> VFIO device is unplugged from the guest.
>
> Hmm, once the page is allocated, then the migratetype is not tracked
> anywhere (except in page_owner debug data). But the unmovable allocations
> might exhaust available unmovable pageblocks and lead to fragmentation. So
> "add MOVABLE back" would be too late. Instead we would need to tell the
>allocator somehow to give us movable page but outside of CMA.

It is it movable, why do we care if it is in CMA or not?

> CMA's own
> __alloc_contig_migrate_range() avoids this problem by allocating movable
> pages, but the range has been already page-isolated and thus the allocator
> won't see the pages there.You obviously can't take this approach and
> isolate all CMA pageblocks like that.  That smells like a new __GFP_FLAG, meh.


I understood (more or less) all of it except the __GFP_FLAG - when/what 
would use it?



>> - do I need to isolate pages by using isolate_migratepages_range,
>> reclaim_clean_pages_from_list like __alloc_contig_migrate_range does?
>> I dropped them for now and the patch uses only @migratepages from
>> the compact_control struct.
>
> You don't have to do reclaim_clean_pages_from_list(), but the isolation has
> to be careful, yeah.


The isolation here means the whole CMA zone isolation which I "obviously 
can't take this approach"? :)


>> - are there any flags in madvise() to address this (could not
>> locate any relevant)?
>
> AFAIK there's no madvise(I_WILL_BE_PINNING_THIS_RANGE)
>
>> - what else is missing? disabled interrupts? locks?
>
> See what isolate_migratepages_block() does.


Thanks for the pointers! I'll have a closer look at Peter's patchset.


-- 
Alexey

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