Re: [PATCH v5 01/13] mm: add zone device coherent type memory support

[Oded Gabbay <oded.gabbay@gmail.com>]

On Fri, Jun 17, 2022 at 8:20 PM Sierra Guiza, Alejandro (Alex)
<alex.sierra@amd.com> wrote:
>
>
> On 6/17/2022 4:40 AM, David Hildenbrand wrote:
> > On 31.05.22 22:00, Alex Sierra wrote:
> >> Device memory that is cache coherent from device and CPU point of view.
> >> This is used on platforms that have an advanced system bus (like CAPI
> >> or CXL). Any page of a process can be migrated to such memory. However,
> >> no one should be allowed to pin such memory so that it can always be
> >> evicted.
> >>
> >> Signed-off-by: Alex Sierra <alex.sierra@amd.com>
> >> Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
> >> Reviewed-by: Alistair Popple <apopple@nvidia.com>
> >> [hch: rebased ontop of the refcount changes,
> >>        removed is_dev_private_or_coherent_page]
> >> Signed-off-by: Christoph Hellwig <hch@lst.de>
> >> ---
> >>   include/linux/memremap.h | 19 +++++++++++++++++++
> >>   mm/memcontrol.c          |  7 ++++---
> >>   mm/memory-failure.c      |  8 ++++++--
> >>   mm/memremap.c            | 10 ++++++++++
> >>   mm/migrate_device.c      | 16 +++++++---------
> >>   mm/rmap.c                |  5 +++--
> >>   6 files changed, 49 insertions(+), 16 deletions(-)
> >>
> >> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> >> index 8af304f6b504..9f752ebed613 100644
> >> --- a/include/linux/memremap.h
> >> +++ b/include/linux/memremap.h
> >> @@ -41,6 +41,13 @@ struct vmem_altmap {
> >>    * A more complete discussion of unaddressable memory may be found in
> >>    * include/linux/hmm.h and Documentation/vm/hmm.rst.
> >>    *
> >> + * MEMORY_DEVICE_COHERENT:
> >> + * Device memory that is cache coherent from device and CPU point of view. This
> >> + * is used on platforms that have an advanced system bus (like CAPI or CXL). A
> >> + * driver can hotplug the device memory using ZONE_DEVICE and with that memory
> >> + * type. Any page of a process can be migrated to such memory. However no one
> > Any page might not be right, I'm pretty sure. ... just thinking about special pages
> > like vdso, shared zeropage, ... pinned pages ...
>
> Hi David,
>
> Yes, I think you're right. This type does not cover all special pages.
> I need to correct that on the cover letter.
> Pinned pages are allowed as long as they're not long term pinned.
>
> Regards,
> Alex Sierra

What if I want to hotplug this device's coherent memory, but I do
*not* want the OS
to migrate any page to it ?
I want to fully-control what resides on this memory, as I can consider
this memory
"expensive". i.e. I don't have a lot of it, I want to use it for
specific purposes and
I don't want the OS to start using it when there is some memory pressure in
the system.

Oded

>
> >
> >> + * should be allowed to pin such memory so that it can always be evicted.
> >> + *
> >>    * MEMORY_DEVICE_FS_DAX:
> >>    * Host memory that has similar access semantics as System RAM i.e. DMA
> >>    * coherent and supports page pinning. In support of coordinating page
> >> @@ -61,6 +68,7 @@ struct vmem_altmap {
> >>   enum memory_type {
> >>      /* 0 is reserved to catch uninitialized type fields */
> >>      MEMORY_DEVICE_PRIVATE = 1,
> >> +    MEMORY_DEVICE_COHERENT,
> >>      MEMORY_DEVICE_FS_DAX,
> >>      MEMORY_DEVICE_GENERIC,
> >>      MEMORY_DEVICE_PCI_P2PDMA,
> >> @@ -143,6 +151,17 @@ static inline bool folio_is_device_private(const struct folio *folio)
> > In general, this LGTM, and it should be correct with PageAnonExclusive I think.
> >
> >
> > However, where exactly is pinning forbidden?
>
> Long-term pinning is forbidden since it would interfere with the device
> memory manager owning the
> device-coherent pages (e.g. evictions in TTM). However, normal pinning
> is allowed on this device type.
>
> Regards,
> Alex Sierra
>
> >