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From:   Alistair Popple <apopple@nvidia.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     "Sierra Guiza, Alejandro (Alex)" <alex.sierra@amd.com>,
        Felix Kuehling <felix.kuehling@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:   Wed, 22 Jun 2022 10:16:27 +1000
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David Hildenbrand <david@redhat.com> writes:

> On 21.06.22 18:08, Sierra Guiza, Alejandro (Alex) wrote:
>>
>> On 6/21/2022 7:25 AM, David Hildenbrand wrote:
>>> On 21.06.22 13:55, Alistair Popple wrote:
>>>> 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:
>>> 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?
>>
>> David, I think you're right. We didn't catch this since the LONGTERM gup
>> test we added to hmm-test only calls to pin_user_pages. Apparently
>> try_grab_folio is called only from fast callers (ex.
>> pin_user_pages_fast/get_user_pages_fast). I have added a conditional
>> similar to what Alistair has proposed to return null on LONGTERM &&
>> (coherent_pages || folio_is_pinnable) at try_grab_folio. Also a new gup
>> test was added with LONGTERM set that calls pin_user_pages_fast.
>> Returning null under this condition it does causes the migration from
>> dev to system memory.
>>
>
> Why can't coherent memory simply put its checks into
> folio_is_pinnable()? I don't get it why we have to do things differently
> here.

I'd made the reasonable assumption that
folio_is_pinnable()/is_pinnable_page() were used to check if the
folio/page is pinnable or not regardless of FOLL_LONGTERM. Looking at
the code more closely though I see both are actually only used on paths
checking for FOLL_LONGTERM pinning.

So I agree - we should rename these
folio_is_longterm_pinnable()/is_longterm_pinnable_page() and add the
check for coherent pages there. Thanks for pointing that out.

 - Alistair

>> Actually, Im having different problems with a call to PageAnonExclusive
>> from try_to_migrate_one during page fault from a HMM test that first
>> migrate pages to device private and forks to mark as COW these pages.
>> Apparently is catching the first BUG VM_BUG_ON_PGFLAGS(!PageAnon(page),
>> page)
>
> With or without this series? A backtrace would be great.