Re: [PATCH hmm 5/5] mm/hmm: remove the customizable pfn format from hmm_range_fault

Jason Gunthorpe
2020-04-29T22:41:31Z

On Wed, Apr 22, 2020 at 01:52:32PM -0400, Felix Kuehling wrote:
> [+Philip Yang]
>
> Am 2020-04-21 um 8:21 p.m. schrieb Jason Gunthorpe:
> > From: Jason Gunthorpe <jgg@mellanox.com>
> >
> > Presumably the intent here was that hmm_range_fault() could put the data
> > into some HW specific format and thus avoid some work. However, nothing
> > actually does that, and it isn't clear how anything actually could do that
> > as hmm_range_fault() provides CPU addresses which must be DMA mapped.
> >
> > Perhaps there is some special HW that does not need DMA mapping, but we
> > don't have any examples of this, and the theoretical performance win of
> > avoiding an extra scan over the pfns array doesn't seem worth the
> > complexity. Plus pfns needs to be scanned anyhow to sort out any
> > DEVICE_PRIVATE pages.
> >
> > This version replaces the uint64_t with an usigned long containing a pfn
> > and fix flags. On input flags is filled with the HMM_PFN_REQ_* values, on
> > successful output it is filled with HMM_PFN_* values, describing the state
> > of the pages.
> >
> > amdgpu is simple to convert, it doesn't use snapshot and doesn't use
> > per-page flags.
> >
> > nouveau uses only 16 hmm_pte entries at most (ie fits in a few cache
> > lines), and it sweeps over its pfns array a couple of times anyhow.
> >
> > Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
> > Signed-off-by: Christoph Hellwig <hch@lst.de>
>
> Hi Jason,
>
> I pointed out a typo in the documentation inline. Other than that, the
> series is
>
> Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
>
> I'll try to build it and run some basic tests later.

Got it, thanks! Let me know if there are problems

Jason