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

David Hildenbrand
2022-06-17T17:33:19Z

On 17.06.22 19:20, Sierra Guiza, Alejandro (Alex) 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 ...
>

Well, you cannot migrate long term pages, that's what I meant :)

>>
>>> + * 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.

I don't see updates to folio_is_pinnable() in this patch.

So wouldn't try_grab_folio() simply pin these pages? What am I missing?

--
Thanks,

David / dhildenb