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

[David Hildenbrand <david@redhat.com>]

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?

I am probably missing something important.

-- 
Thanks,

David / dhildenb