Re: [PATCH v5 01/13] mm: add zone device coherent type memory support

David Hildenbrand
2022-06-21T12:25:31Z

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?

--
Thanks,

David / dhildenb