Date:   Wed, 22 Jun 2022 18:06:10 -0500
From:   "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>
To:     Alistair Popple <apopple@...dia.com>,
        David Hildenbrand <david@...hat.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 7:16 PM, Alistair Popple wrote:
> David Hildenbrand <david@...hat.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@...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.
> 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

Will do in the next patch series.

Regards,
Alex Sierra

>>> 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.

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