linux-ext4 - Re: [PATCH v5 01/13] mm: add zone device coherent type memory support

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Message-ID: <7605beee-0a76-4ee9-e950-17419630f2cf@redhat.com>
Date:   Fri, 17 Jun 2022 19:33:12 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>,
        jgg@...dia.com
Cc:     Felix.Kuehling@....com, linux-mm@...ck.org, rcampbell@...dia.com,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        hch@....de, jglisse@...hat.com, apopple@...dia.com,
        willy@...radead.org, akpm@...ux-foundation.org
Subject: Re: [PATCH v5 01/13] mm: add zone device coherent type memory support

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@....com>
>>> Acked-by: Felix Kuehling <Felix.Kuehling@....com>
>>> Reviewed-by: Alistair Popple <apopple@...dia.com>
>>> [hch: rebased ontop of the refcount changes,
>>>        removed is_dev_private_or_coherent_page]
>>> Signed-off-by: Christoph Hellwig <hch@....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