Date:   Thu, 23 Jun 2022 20:21:54 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>,
        Alistair Popple <apopple@...dia.com>, akpm@...ux-foundation.org
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
Subject: Re: [PATCH v5 01/13] mm: add zone device coherent type memory support

On 23.06.22 20:20, Sierra Guiza, Alejandro (Alex) wrote:
> 
> On 6/23/2022 2:57 AM, David Hildenbrand wrote:
>> On 23.06.22 01:16, Sierra Guiza, Alejandro (Alex) wrote:
>>> On 6/21/2022 11:16 AM, David Hildenbrand wrote:
>>>> 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@...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.
>>>>>
>>>> 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.
>>>>
>>>>> 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.
>>> Here's the back trace. This happens in a hmm-test added in this patch
>>> series. However, I have tried to isolate this BUG by just adding the COW
>>> test with private device memory only. This is only present as follows.
>>> Allocate anonymous mem->Migrate to private device memory->fork->try to
>>> access to parent's anonymous memory (which will suppose to trigger a
>>> page fault and migration to system mem). Just for the record, if the
>>> child is terminated before the parent's memory is accessed, this problem
>>> is not present.
>>
>> The only usage of PageAnonExclusive() in try_to_migrate_one() is:
>>
>> anon_exclusive = folio_test_anon(folio) &&
>> 		 PageAnonExclusive(subpage);
>>
>> Which can only possibly fail if subpage is not actually part of the folio.
>>
>>
>> I see some controversial code in the the if (folio_is_zone_device(folio)) case later:
>>
>> 			 * The assignment to subpage above was computed from a
>> 			 * swap PTE which results in an invalid pointer.
>> 			 * Since only PAGE_SIZE pages can currently be
>> 			 * migrated, just set it to page. This will need to be
>> 			 * changed when hugepage migrations to device private
>> 			 * memory are supported.
>> 			 */
>> 			subpage = &folio->page;
>>
>> There we have our invalid pointer hint.
>>
>> I don't see how it could have worked if the child quit, though? Maybe
>> just pure luck?
>>
>>
>> Does the following fix your issue:
> 
> Yes, it fixed the issue. Thanks. Should we include this patch in this 
> patch series or separated?
> 
> Regards,
> Alex Sierra

I'll send it right away "officially" so we can get it into 5.19. Can I
add your tested-by?


-- 
Thanks,

David / dhildenb

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