RE: [RFC PATCH v3 1/2] mempinfd: Add new syscall to provide memory pin

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: owner-linux-mm@xxxxxxxxx [mailto:owner-linux-mm@xxxxxxxxx] On Behalf Of
> David Hildenbrand
> Sent: Monday, February 8, 2021 9:22 PM
> To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>; Matthew Wilcox
> <willy@xxxxxxxxxxxxx>
> Cc: Wangzhou (B) <wangzhou1@xxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx;
> iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx;
> linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux-api@xxxxxxxxxxxxxxx; Andrew
> Morton <akpm@xxxxxxxxxxxxxxxxxxxx>; Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>;
> gregkh@xxxxxxxxxxxxxxxxxxx; jgg@xxxxxxxx; kevin.tian@xxxxxxxxx;
> jean-philippe@xxxxxxxxxx; eric.auger@xxxxxxxxxx; Liguozhu (Kenneth)
> <liguozhu@xxxxxxxxxxxxx>; zhangfei.gao@xxxxxxxxxx; chensihang (A)
> <chensihang1@xxxxxxxxxxxxx>
> Subject: Re: [RFC PATCH v3 1/2] mempinfd: Add new syscall to provide memory
> pin
> 
> On 08.02.21 03:27, Song Bao Hua (Barry Song) wrote:
> >
> >
> >> -----Original Message-----
> >> From: owner-linux-mm@xxxxxxxxx [mailto:owner-linux-mm@xxxxxxxxx] On Behalf
> Of
> >> Matthew Wilcox
> >> Sent: Monday, February 8, 2021 2:31 PM
> >> To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>
> >> Cc: Wangzhou (B) <wangzhou1@xxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx;
> >> iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx;
> >> linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux-api@xxxxxxxxxxxxxxx; Andrew
> >> Morton <akpm@xxxxxxxxxxxxxxxxxxxx>; Alexander Viro
> <viro@xxxxxxxxxxxxxxxxxx>;
> >> gregkh@xxxxxxxxxxxxxxxxxxx; jgg@xxxxxxxx; kevin.tian@xxxxxxxxx;
> >> jean-philippe@xxxxxxxxxx; eric.auger@xxxxxxxxxx; Liguozhu (Kenneth)
> >> <liguozhu@xxxxxxxxxxxxx>; zhangfei.gao@xxxxxxxxxx; chensihang (A)
> >> <chensihang1@xxxxxxxxxxxxx>
> >> Subject: Re: [RFC PATCH v3 1/2] mempinfd: Add new syscall to provide memory
> >> pin
> >>
> >> On Sun, Feb 07, 2021 at 10:24:28PM +0000, Song Bao Hua (Barry Song) wrote:
> >>>>> In high-performance I/O cases, accelerators might want to perform
> >>>>> I/O on a memory without IO page faults which can result in dramatically
> >>>>> increased latency. Current memory related APIs could not achieve this
> >>>>> requirement, e.g. mlock can only avoid memory to swap to backup device,
> >>>>> page migration can still trigger IO page fault.
> >>>>
> >>>> Well ... we have two requirements.  The application wants to not take
> >>>> page faults.  The system wants to move the application to a different
> >>>> NUMA node in order to optimise overall performance.  Why should the
> >>>> application's desires take precedence over the kernel's desires?  And why
> >>>> should it be done this way rather than by the sysadmin using numactl to
> >>>> lock the application to a particular node?
> >>>
> >>> NUMA balancer is just one of many reasons for page migration. Even one
> >>> simple alloc_pages() can cause memory migration in just single NUMA
> >>> node or UMA system.
> >>>
> >>> The other reasons for page migration include but are not limited to:
> >>> * memory move due to CMA
> >>> * memory move due to huge pages creation
> >>>
> >>> Hardly we can ask users to disable the COMPACTION, CMA and Huge Page
> >>> in the whole system.
> >>
> >> You're dodging the question.  Should the CMA allocation fail because
> >> another application is using SVA?
> >>
> >> I would say no.
> >
> > I would say no as well.
> >
> > While IOMMU is enabled, CMA almost has one user only: IOMMU driver
> > as other drivers will depend on iommu to use non-contiguous memory
> > though they are still calling dma_alloc_coherent().
> >
> > In iommu driver, dma_alloc_coherent is called during initialization
> > and there is no new allocation afterwards. So it wouldn't cause
> > runtime impact on SVA performance. Even there is new allocations,
> > CMA will fall back to general alloc_pages() and iommu drivers are
> > almost allocating small memory for command queues.
> >
> > So I would say general compound pages, huge pages, especially
> > transparent huge pages, would be bigger concerns than CMA for
> > internal page migration within one NUMA.
> >
> > Not like CMA, general alloc_pages() can get memory by moving
> > pages other than those pinned.
> >
> > And there is no guarantee we can always bind the memory of
> > SVA applications to single one NUMA, so NUMA balancing is
> > still a concern.
> >
> > But I agree we need a way to make CMA success while the userspace
> > pages are pinned. Since pin has been viral in many drivers, I
> > assume there is a way to handle this. Otherwise, APIs like
> > V4L2_MEMORY_USERPTR[1] will possibly make CMA fail as there
> > is no guarantee that usersspace will allocate unmovable memory
> > and there is no guarantee the fallback path- alloc_pages() can
> > succeed while allocating big memory.
> >
> 
> Long term pinnings cannot go onto CMA-reserved memory, and there is
> similar work to also fix ZONE_MOVABLE in that regard.
> 
> https://lkml.kernel.org/r/20210125194751.1275316-1-pasha.tatashin@soleen.c
> om
> 
> One of the reasons I detest using long term pinning of pages where it
> could be avoided. Take VFIO and RDMA as an example: these things
> currently can't work without them.
> 
> What I read here: "DMA performance will be affected severely". That does
> not sound like a compelling argument to me for long term pinnings.
> Please find another way to achieve the same goal without long term
> pinnings controlled by user space - e.g., controlling when migration
> actually happens.
> 
> For example, CMA/alloc_contig_range()/memory unplug are corner cases
> that happen rarely, you shouldn't have to worry about them messing with
> your DMA performance.

I agree CMA/alloc_contig_range()/memory unplug would be corner cases,
the major cases would be THP, NUMA balancing while we could totally
disable them but it seems insensible to do that only because there is
a process using SVA in the system.

> 
> --
> Thanks,
> 
> David / dhildenb

Thanks
Barry