linux-kernel - Re: [PATCH v2 2/5] mm/migrate: add a direction parameter to migrate

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Message-ID: <20200720195943.GH2021234@nvidia.com>
Date:   Mon, 20 Jul 2020 16:59:43 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Ralph Campbell <rcampbell@...dia.com>
CC:     <linux-rdma@...r.kernel.org>, <linux-mm@...ck.org>,
        <nouveau@...ts.freedesktop.org>, <kvm-ppc@...r.kernel.org>,
        <linux-kselftest@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Jerome Glisse <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>, Ben Skeggs <bskeggs@...hat.com>,
        Bharata B Rao <bharata@...ux.ibm.com>
Subject: Re: [PATCH v2 2/5] mm/migrate: add a direction parameter to
 migrate_vma

On Mon, Jul 20, 2020 at 12:54:53PM -0700, Ralph Campbell wrote:
> > > diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> > > index 3e546cbf03dd..620f2235d7d4 100644
> > > +++ b/include/linux/migrate.h
> > > @@ -180,6 +180,11 @@ static inline unsigned long migrate_pfn(unsigned long pfn)
> > >   	return (pfn << MIGRATE_PFN_SHIFT) | MIGRATE_PFN_VALID;
> > >   }
> > > +enum migrate_vma_direction {
> > > +	MIGRATE_VMA_FROM_SYSTEM,
> > > +	MIGRATE_VMA_FROM_DEVICE_PRIVATE,
> > > +};
> > 
> > I would have guessed this is more natural as _FROM_DEVICE_ and
> > TO_DEVICE_ ?
> 
> The caller controls where the destination memory is allocated so it isn't
> necessarily device private memory, it could be from system to system.
> The use case for system to system memory migration is for hardware
> like ARM SMMU or PCIe ATS where a single set of page tables is shared by
> the device and a CPU process over a coherent system memory bus.
> Also many integrated GPUs in SOCs fall into this category too.

Maybe just TO/FROM_DEIVCE then? Even though the memory is not
DEVICE_PRIVATE it is still device owned pages right?

> So to me, it makes more sense to specify the direction based on the
> source location.

It feels strange because the driver doesn't always know or control the
source?

Jason