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

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Date:   Fri, 17 Jun 2022 14:27:43 -0500
From:   "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>
To:     David Hildenbrand <david@...hat.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 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@....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.
Device coherent type pages should return true here, as they are pinnable 
pages.
>
> 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.

Regards,
Alex Sierra

>