Re: [PATCH v1 08/12] mm/rmap: handle device-exclusive entries correctly in try_to_unmap_one()

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Jan 30, 2025 at 02:06:12PM +0100, Simona Vetter wrote:
> On Thu, Jan 30, 2025 at 12:08:42PM +0100, David Hildenbrand wrote:
> > On 30.01.25 11:10, Simona Vetter wrote:
> > > On Wed, Jan 29, 2025 at 12:54:06PM +0100, David Hildenbrand wrote:
> > > > Ever since commit b756a3b5e7ea ("mm: device exclusive memory access")
> > > > we can return with a device-exclusive entry from page_vma_mapped_walk().
> > > > 
> > > > try_to_unmap_one() is not prepared for that, so teach it about these
> > > > non-present nonswap PTEs.
> > > > 
> > > > Before that, could we also have triggered this case with device-private
> > > > entries? Unlikely.
> > > 
> > > Just quick comment on this, I'm still pondering all the other aspects.
> > > 
> > > device-private memory is entirely owned by the driver, the core mm isn't
> > > supposed to touch these beyond migrating it back to system memory in
> > > do_swap_page. Plus using rmap when the driver asks for invalidating
> > > mappings as needed.
> > > 
> > > So no lru, thp, migration or anything initiated by core mm should ever
> > > happen on these device private pages. If it does, it'd be a bug.
> > 
> > I was not 100% sure about HWPoison handling, that's why I added that
> > comment. In other regards I agree: reclaim etc. does not apply.
> 
> So maybe I'm just entirely lost, but unless you have a coherent
> interconnect I don't think hwpoisin should get involved with device
> private memory? And for a coherent interconnect it's just device memory,
> which isn't treated very special.

I'm not sure it is meaningful, but in principle a driver could keep
track of the poisoned private memory using that struct page
bit. Perhaps in that sense it is more of a driver private flag than
something the core MM would touch.

If you have a coherent interconnect then you should not be using
device private.

Jason