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From:   Jason Gunthorpe <jgg@mellanox.com>
To:     Jerome Glisse <jglisse@redhat.com>
CC:     Dan Williams <dan.j.williams@intel.com>,
        Christoph Hellwig <hch@lst.de>,
        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>
Subject: Re: [PATCH 04/15] mm: remove the pgmap field from struct hmm_vma_walk
Thread-Topic: [PATCH 04/15] mm: remove the pgmap field from struct
 hmm_vma_walk
Thread-Index: AQHVTHDc5B4IgstYQk6yBJaVfn8xGqbv9wIAgAARNACAAMySgIAJE76AgABlPQCAAGF5AIAAFowAgAHIzYCAABXxgA==
Date:   Thu, 15 Aug 2019 19:22:02 +0000
Message-ID: <20190815192157.GB22970@mellanox.com>
References: <20190806160554.14046-1-hch@lst.de>
 <20190806160554.14046-5-hch@lst.de> <20190807174548.GJ1571@mellanox.com>
 <CAPcyv4hPCuHBLhSJgZZEh0CbuuJNPLFDA3f-79FX5uVOO0yubA@mail.gmail.com>
 <20190808065933.GA29382@lst.de>
 <CAPcyv4hMUzw8vyXFRPe2pdwef0npbMm9tx9wiZ9MWkHGhH1V6w@mail.gmail.com>
 <20190814073854.GA27249@lst.de> <20190814132746.GE13756@mellanox.com>
 <CAPcyv4g8usp8prJ+1bMtyV1xuedp5FKErBp-N8+KzR=rJ-v0QQ@mail.gmail.com>
 <20190815180325.GA4920@redhat.com>
In-Reply-To: <20190815180325.GA4920@redhat.com>
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On Thu, Aug 15, 2019 at 02:03:25PM -0400, Jerome Glisse wrote:
> On Wed, Aug 14, 2019 at 07:48:28AM -0700, Dan Williams wrote:
> > On Wed, Aug 14, 2019 at 6:28 AM Jason Gunthorpe <jgg@mellanox.com> wrot=
e:
> > >
> > > 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 exa=
mple
> > > > > I can think of a PMD not containing a uniform pgmap association f=
or
> > > > > each pte is the case when the pgmap overlaps normal dram, i.e. sh=
ares
> > > > > the same 'struct memory_section' for a given span. Otherwise, dis=
tinct
> > > > > 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 "pf=
n is
> > > > > ZONE_DEVICE".
> > > >
> > > > So I guess this patch is fine for now, and once you provide a bette=
r
> > > > 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?
> >=20
> > Yes, if the scan is over a single pmd then caching it makes sense.
>=20
> Quite frankly an easier an better solution is to remove the pagemap
> lookup as HMM user abide by mmu notifier it means we will not make
> use or dereference the struct page so that we are safe from any
> racing hotunplug of dax memory (as long as device driver using hmm
> do not have a bug).

Yes, I also would prefer to drop the confusing checks entirely -
Christoph can you resend this patch?

Thanks,
Jason=20