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Re: [PATCH v11 07/14] HMM: mm add helper to update page table when migrating memory v2.

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  Re: [PATCH v11 07/14] HMM: mm add helper to update page table when migrating memory v2. "Hillf Danton" <hillf.zj@alibaba-inc.com> - 2015-10-22 11:50 +0200
    Re: [PATCH v11 07/14] HMM: mm add helper to update page table when  migrating memory v2. Jerome Glisse <jglisse@redhat.com> - 2015-10-22 16:30 +0200
      Re: [PATCH v11 07/14] HMM: mm add helper to update page table when migrating memory v2. "Hillf Danton" <hillf.zj@alibaba-inc.com> - 2015-10-23 05:30 +0200

#1253683 — Re: [PATCH v11 07/14] HMM: mm add helper to update page table when migrating memory v2.

> 
> This is a multi-stage process, first we save and replace page table
> entry with special HMM entry, also flushing tlb in the process. If
> we run into non allocated entry we either use the zero page or we
> allocate new page. For swaped entry we try to swap them in.
> 
Please elaborate why swap entry is handled this way.


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#1253875 — Re: [PATCH v11 07/14] HMM: mm add helper to update page table when migrating memory v2.

On Thu, Oct 22, 2015 at 05:46:47PM +0800, Hillf Danton wrote:
> > 
> > This is a multi-stage process, first we save and replace page table
> > entry with special HMM entry, also flushing tlb in the process. If
> > we run into non allocated entry we either use the zero page or we
> > allocate new page. For swaped entry we try to swap them in.
> > 
> Please elaborate why swap entry is handled this way.

So first, this is only when you have a device then use HMM and a device
that use memory migration. So far it only make sense for discrete GPUs.
So regular workload that do not use a GPUs with HMM are not impacted and
will not go throught this code path.

Now, here we are migrating memory because the device driver is asking for
it, so presumably we are expecting that the device will use that memory
hence we want to swap in anything that have been swap to disk. Once it is
swap in memory we copy it to device memory and free the pages. So in the
end we only need to allocate a page temporarily until we move things to
the device.

Cheers,
Jérôme
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#1254308

> > > This is a multi-stage process, first we save and replace page table
> > > entry with special HMM entry, also flushing tlb in the process. If
> > > we run into non allocated entry we either use the zero page or we
> > > allocate new page. For swaped entry we try to swap them in.
> > >
> > Please elaborate why swap entry is handled this way.
> 
> So first, this is only when you have a device then use HMM and a device
> that use memory migration. So far it only make sense for discrete GPUs.
> So regular workload that do not use a GPUs with HMM are not impacted and
> will not go throught this code path.
> 
> Now, here we are migrating memory because the device driver is asking for
> it, so presumably we are expecting that the device will use that memory
> hence we want to swap in anything that have been swap to disk. Once it is
> swap in memory we copy it to device memory and free the pages. So in the
> end we only need to allocate a page temporarily until we move things to
> the device.
> 
I prefer it is in log message.

thanks
Hillf

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