linux-kernel - Re: [RESEND PATCH 2/3] nouveau: fix mixed normal and device private page migration

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Date:   Thu, 25 Jun 2020 14:31:44 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Ralph Campbell <rcampbell@...dia.com>
Cc:     Christoph Hellwig <hch@....de>, nouveau@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, Jerome Glisse <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Ben Skeggs <bskeggs@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Bharata B Rao <bharata@...ux.ibm.com>
Subject: Re: [RESEND PATCH 2/3] nouveau: fix mixed normal and device private
 page migration

On Thu, Jun 25, 2020 at 10:25:38AM -0700, Ralph Campbell wrote:
> Making sure to include linux-mm and Bharata B Rao for IBM's
> use of migrate_vma*().
> 
> On 6/24/20 11:10 AM, Ralph Campbell wrote:
> > 
> > On 6/24/20 12:23 AM, Christoph Hellwig wrote:
> > > On Mon, Jun 22, 2020 at 04:38:53PM -0700, Ralph Campbell wrote:
> > > > The OpenCL function clEnqueueSVMMigrateMem(), without any flags, will
> > > > migrate memory in the given address range to device private memory. The
> > > > source pages might already have been migrated to device private memory.
> > > > In that case, the source struct page is not checked to see if it is
> > > > a device private page and incorrectly computes the GPU's physical
> > > > address of local memory leading to data corruption.
> > > > Fix this by checking the source struct page and computing the correct
> > > > physical address.
> > > 
> > > I'm really worried about all this delicate code to fix the mixed
> > > ranges.  Can't we make it clear at the migrate_vma_* level if we want
> > > to migrate from or two device private memory, and then skip all the work
> > > for regions of memory that already are in the right place?  This might be
> > > a little more work initially, but I think it leads to a much better
> > > API.
> > > 
> > 
> > The current code does encode the direction with src_owner != NULL meaning
> > device private to system memory and src_owner == NULL meaning system
> > memory to device private memory. This patch would obviously defeat that
> > so perhaps a flag could be added to the struct migrate_vma to indicate the
> > direction but I'm unclear how that makes things less delicate.
> > Can you expand on what you are worried about?
> > 
> > The issue with invalidations might be better addressed by letting the device
> > driver handle device private page TLB invalidations when migrating to
> > system memory and changing migrate_vma_setup() to only invalidate CPU
> > TLB entries for normal pages being migrated to device private memory.
> > If a page isn't migrating, it seems inefficient to invalidate those TLB
> > entries.
> > 
> > Any other suggestions?
> 
> After a night's sleep, I think this might work. What do others think?
> 
> 1) Add a new MMU_NOTIFY_MIGRATE enum to mmu_notifier_event.
> 
> 2) Change migrate_vma_collect() to use the new MMU_NOTIFY_MIGRATE event type.
>
> 3) Modify nouveau_svmm_invalidate_range_start() to simply return (no invalidations)
> for MMU_NOTIFY_MIGRATE mmu notifier callbacks.

Isn't it a bit of an assumption that migrate_vma_collect() is only
used by nouveau itself?

What if some other devices' device_private pages are being migrated?

Jason