linux-kernel - Re: [PATCH hmm 5/5] mm/hmm: remove the customizable pfn format from hmm_range

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Message-ID: <20200429224127.GB3824@ziepe.ca>
Date:   Wed, 29 Apr 2020 19:41:27 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Felix Kuehling <felix.kuehling@....com>
Cc:     linux-mm@...ck.org, Ralph Campbell <rcampbell@...dia.com>,
        "Yang, Philip" <Philip.Yang@....com>,
        Alex Deucher <alexander.deucher@....com>,
        amd-gfx@...ts.freedesktop.org, Ben Skeggs <bskeggs@...hat.com>,
        Christian König <christian.koenig@....com>,
        "David (ChunMing) Zhou" <David1.Zhou@....com>,
        dri-devel@...ts.freedesktop.org, Christoph Hellwig <hch@....de>,
        intel-gfx@...ts.freedesktop.org,
        Jérôme Glisse <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        linux-kernel@...r.kernel.org,
        Niranjana Vishwanathapura <niranjana.vishwanathapura@...el.com>,
        nouveau@...ts.freedesktop.org
Subject: Re: [PATCH hmm 5/5] mm/hmm: remove the customizable pfn format from
 hmm_range_fault

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@...lanox.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@...lanox.com>
> > Signed-off-by: Christoph Hellwig <hch@....de>
> 
> Hi Jason,
> 
> I pointed out a typo in the documentation inline. Other than that, the
> series is
> 
> Acked-by: Felix Kuehling <Felix.Kuehling@....com>
> 
> I'll try to build it and run some basic tests later.

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

Jason