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From:   Alistair Popple <apopple@nvidia.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Felix Kuehling <felix.kuehling@amd.com>,
        "Sierra Guiza, Alejandro (Alex)" <alex.sierra@amd.com>,
        jgg@nvidia.com, linux-mm@kvack.org, rcampbell@nvidia.com,
        linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org,
        amd-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
        hch@lst.de, jglisse@redhat.com, willy@infradead.org,
        akpm@linux-foundation.org
Subject: Re: [PATCH v5 01/13] mm: add zone device coherent type memory support
Date:   Tue, 21 Jun 2022 21:55:24 +1000
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David Hildenbrand <david@redhat.com> writes:

> 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().

Ugh, I think you might be right about try_grab_folio().

We didn't update folio_is_pinnable() to include device coherent pages
because device coherent pages are pinnable. It is really just
FOLL_LONGTERM that we want to prevent here.

For normal PUP that is done by my change in
check_and_migrate_movable_pages() which migrates pages being pinned with
FOLL_LONGTERM. But I think I incorrectly assumed we would take the
pte_devmap() path in gup_pte_range(), which we don't for coherent pages.
So I think the check in try_grab_folio() needs to be:

 		if (unlikely((flags & FOLL_LONGTERM) &&
 			     (!is_pinnable_page(page) || is_device_coherent_page(page))))

 - Alistair