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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Wed, 14 Aug 2019 07:48:28 -0700
Message-ID: <CAPcyv4g8usp8prJ+1bMtyV1xuedp5FKErBp-N8+KzR=rJ-v0QQ@mail.gmail.com>
Subject: Re: [PATCH 04/15] mm: remove the pgmap field from struct hmm_vma_walk
To:     Jason Gunthorpe <jgg@mellanox.com>
Cc:     Christoph Hellwig <hch@lst.de>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        Ben Skeggs <bskeggs@redhat.com>,
        Felix Kuehling <Felix.Kuehling@amd.com>,
        Ralph Campbell <rcampbell@nvidia.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "nouveau@lists.freedesktop.org" <nouveau@lists.freedesktop.org>,
        "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
        "amd-gfx@lists.freedesktop.org" <amd-gfx@lists.freedesktop.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Wed, Aug 14, 2019 at 6:28 AM Jason Gunthorpe <jgg@mellanox.com> wrote:
>
> On Wed, Aug 14, 2019 at 09:38:54AM +0200, Christoph Hellwig wrote:
> > On Tue, Aug 13, 2019 at 06:36:33PM -0700, Dan Williams wrote:
> > > Section alignment constraints somewhat save us here. The only example
> > > I can think of a PMD not containing a uniform pgmap association for
> > > each pte is the case when the pgmap overlaps normal dram, i.e. shares
> > > the same 'struct memory_section' for a given span. Otherwise, distinct
> > > pgmaps arrange to manage their own exclusive sections (and now
> > > subsections as of v5.3). Otherwise the implementation could not
> > > guarantee different mapping lifetimes.
> > >
> > > That said, this seems to want a better mechanism to determine "pfn is
> > > ZONE_DEVICE".
> >
> > So I guess this patch is fine for now, and once you provide a better
> > mechanism we can switch over to it?
>
> What about the version I sent to just get rid of all the strange
> put_dev_pagemaps while scanning? Odds are good we will work with only
> a single pagemap, so it makes some sense to cache it once we find it?

Yes, if the scan is over a single pmd then caching it makes sense.