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

Sierra Guiza, Alejandro (Alex)
2022-06-17T17:20:55Z

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

>
>> + * 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

>