Date:   Tue, 21 Jun 2022 13:25:32 +0200
From:   Felix Kuehling <felix.kuehling@....com>
To:     David Hildenbrand <david@...hat.com>,
        "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>,
        jgg@...dia.com
Cc:     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, apopple@...dia.com,
        willy@...radead.org, akpm@...ux-foundation.org
Subject: Re: [PATCH v5 01/13] mm: add zone device coherent type memory support


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").

Regards,
   Felix


>
> I am probably missing something important.
>
P.S.: I'm on vacation and looking at a tiny screen. Hope I didn't miss 
anything myself.

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