Message-ID: <643c44e7-48be-375b-c7ab-6a30b5ee2937@redhat.com>
Date:   Tue, 21 Jun 2022 14:25:25 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     Felix Kuehling <felix.kuehling@....com>,
        "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>,
        jgg@...dia.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, willy@...radead.org,
        akpm@...ux-foundation.org
Subject: Re: [PATCH v5 01/13] mm: add zone device coherent type memory support

On 21.06.22 13:55, Alistair Popple wrote:
> 
> David Hildenbrand <david@...hat.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@....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.
>>>> 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:

I think I said it already (and I might be wrong without reading the
code), but folio_is_pinnable() is *only* called for long-term pinnings.

It should actually be called folio_is_longterm_pinnable().

That's where that check should go, no?

-- 
Thanks,

David / dhildenb

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