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

[David Hildenbrand <david@redhat.com>]

On 21.06.22 13:25, Felix Kuehling wrote:
> 
> Am 6/17/22 um 23:19 schrieb David Hildenbrand:
>> On 17.06.22 21:27, Sierra Guiza, Alejandro (Alex) wrote:
>>> On 6/17/2022 12:33 PM, David Hildenbrand wrote:
>>>> 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.
>>> Device coherent type pages should return true here, as they are pinnable
>>> pages.
>> That function is only called for long-term pinnings in try_grab_folio().
>>
>>>> So wouldn't try_grab_folio() simply pin these pages? What am I missing?
>>> As far as I understand this return NULL for long term pin pages.
>>> Otherwise they get refcount incremented.
>> I don't follow.
>>
>> You're saying
>>
>> a) folio_is_pinnable() returns true for device coherent pages
>>
>> and that
>>
>> b) device coherent pages don't get long-term pinned
>>
>>
>> Yet, the code says
>>
>> struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags)
>> {
>> 	if (flags & FOLL_GET)
>> 		return try_get_folio(page, refs);
>> 	else if (flags & FOLL_PIN) {
>> 		struct folio *folio;
>>
>> 		/*
>> 		 * Can't do FOLL_LONGTERM + FOLL_PIN gup fast path if not in a
>> 		 * right zone, so fail and let the caller fall back to the slow
>> 		 * path.
>> 		 */
>> 		if (unlikely((flags & FOLL_LONGTERM) &&
>> 			     !is_pinnable_page(page)))
>> 			return NULL;
>> 		...
>> 		return folio;
>> 	}
>> }
>>
>>
>> What prevents these pages from getting long-term pinned as stated in this patch?
> 
> Long-term pinning is handled by __gup_longterm_locked, which migrates 
> pages returned by __get_user_pages_locked that cannot be long-term 
> pinned. try_grab_folio is OK to grab the pages. Anything that can't be 
> long-term pinned will be migrated afterwards, and 
> __get_user_pages_locked will be retried. The migration of 
> DEVICE_COHERENT pages was implemented by Alistair in patch 5/13 
> ("mm/gup: migrate device coherent pages when pinning instead of failing").

Thanks.

__gup_longterm_locked()->check_and_migrate_movable_pages()

Which checks folio_is_pinnable() and doesn't do anything if set.

Sorry to be dense here, but I don't see how what's stated in this patch
works without adjusting folio_is_pinnable().

-- 
Thanks,

David / dhildenb