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

[Alistair Popple <apopple@nvidia.com>]

Oded Gabbay <oded.gabbay@gmail.com> writes:

> 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.

This is exactly what MEMORY_DEVICE_COHERENT is for. Device coherent
pages are only allocated by a device driver and exposed to user-space by
a driver migrating pages to them with migrate_vma. The OS can't just
start using them due to memory pressure for example.

 - Alistair

> 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
>>
>> >