Date:   Tue, 21 Jun 2022 11:08:42 -0500
From:   "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>
To:     David Hildenbrand <david@...hat.com>,
        Alistair Popple <apopple@...dia.com>
Cc:     Felix Kuehling <felix.kuehling@....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 6/21/2022 7:25 AM, David Hildenbrand wrote:
> 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?

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.

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)

Regards,
Alex Sierra

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