[PATCH] mm/device-public-memory: Enable move_pages() to stat device memory

[PATCH] mm/device-public-memory: Enable move_pages() to stat device memory

[Reza Arbab]

The move_pages() syscall can be used to find the numa node where a page
currently resides. This is not working for device public memory pages,
which erroneously report -EFAULT (unmapped or zero page).

Enable by adding a FOLL_DEVICE flag for follow_page(), which
move_pages() will use. This could be done unconditionally, but adding a
flag seems like a safer change.

Cc: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Reza Arbab <arbab@linux.vnet.ibm.com>
---
 include/linux/mm.h | 1 +
 mm/gup.c           | 2 +-
 mm/migrate.c       | 2 +-
 3 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index f8c10d3..783cb57 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2368,6 +2368,7 @@ static inline struct page *follow_page(struct vm_area_struct *vma,
 #define FOLL_MLOCK	0x1000	/* lock present pages */
 #define FOLL_REMOTE	0x2000	/* we are working on non-current tsk/mm */
 #define FOLL_COW	0x4000	/* internal GUP flag */
+#define FOLL_DEVICE	0x8000	/* return device pages */
 
 static inline int vm_fault_to_errno(int vm_fault, int foll_flags)
 {
diff --git a/mm/gup.c b/mm/gup.c
index b2b4d42..6fbad70 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -110,7 +110,7 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
 		return NULL;
 	}
 
-	page = vm_normal_page(vma, address, pte);
+	page = _vm_normal_page(vma, address, pte, flags & FOLL_DEVICE);
 	if (!page && pte_devmap(pte) && (flags & FOLL_GET)) {
 		/*
 		 * Only return device mapping pages in the FOLL_GET case since
diff --git a/mm/migrate.c b/mm/migrate.c
index 6954c14..dea0ceb 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1690,7 +1690,7 @@ static void do_pages_stat_array(struct mm_struct *mm, unsigned long nr_pages,
 			goto set_status;
 
 		/* FOLL_DUMP to ignore special (like zero) pages */
-		page = follow_page(vma, addr, FOLL_DUMP);
+		page = follow_page(vma, addr, FOLL_DUMP | FOLL_DEVICE);
 
 		err = PTR_ERR(page);
 		if (IS_ERR(page))
-- 
1.8.3.1

Re: [PATCH] mm/device-public-memory: Enable move_pages() to stat device memory

[Reza Arbab]

On Fri, Sep 22, 2017 at 08:13:56PM +0000, Reza Arbab wrote:
>The move_pages() syscall can be used to find the numa node where a page
>currently resides. This is not working for device public memory pages,
>which erroneously report -EFAULT (unmapped or zero page).

Argh. Please disregard this patch.

My test setup has a chunk of system memory carved out as pretend device 
public memory, to experiment with. Of course the real thing has no numa 
node!

Apologies all, it's been a long day.

-- 
Reza Arbab

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Re: [PATCH] mm/device-public-memory: Enable move_pages() to stat device memory

[Reza Arbab]

On Fri, Sep 22, 2017 at 08:31:57PM +0000, Reza Arbab wrote:
>On Fri, Sep 22, 2017 at 08:13:56PM +0000, Reza Arbab wrote:
>>The move_pages() syscall can be used to find the numa node where a page
>>currently resides. This is not working for device public memory pages,
>>which erroneously report -EFAULT (unmapped or zero page).
>
>Argh. Please disregard this patch.
>
>My test setup has a chunk of system memory carved out as pretend 
>device public memory, to experiment with. Of course the real thing has 
>no numa node!

On third thought, yes it does! 

static int hmm_devmem_pages_create(struct hmm_devmem *devmem)
{
	:
	nid = dev_to_node(device);
	if (nid < 0)
		nid = numa_mem_id();
	:
	if (devmem->pagemap.type == MEMORY_DEVICE_PUBLIC)
		ret = arch_add_memory(nid, align_start, align_size, false);
	:
}

So now I think the patch may be right after all. Please un-disregard it.  
Regard it? Whatever.

-- 
Reza Arbab

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Re: [PATCH] mm/device-public-memory: Enable move_pages() to stat device memory

[Michal Hocko]

On Fri 22-09-17 15:13:56, Reza Arbab wrote:
> The move_pages() syscall can be used to find the numa node where a page
> currently resides. This is not working for device public memory pages,
> which erroneously report -EFAULT (unmapped or zero page).
> 
> Enable by adding a FOLL_DEVICE flag for follow_page(), which
> move_pages() will use. This could be done unconditionally, but adding a
> flag seems like a safer change.

I do not understand purpose of this patch. What is the numa node of a
device memory?

> Cc: Jerome Glisse <jglisse@redhat.com>
> Signed-off-by: Reza Arbab <arbab@linux.vnet.ibm.com>
> ---
>  include/linux/mm.h | 1 +
>  mm/gup.c           | 2 +-
>  mm/migrate.c       | 2 +-
>  3 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index f8c10d3..783cb57 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2368,6 +2368,7 @@ static inline struct page *follow_page(struct vm_area_struct *vma,
>  #define FOLL_MLOCK	0x1000	/* lock present pages */
>  #define FOLL_REMOTE	0x2000	/* we are working on non-current tsk/mm */
>  #define FOLL_COW	0x4000	/* internal GUP flag */
> +#define FOLL_DEVICE	0x8000	/* return device pages */
>  
>  static inline int vm_fault_to_errno(int vm_fault, int foll_flags)
>  {
> diff --git a/mm/gup.c b/mm/gup.c
> index b2b4d42..6fbad70 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -110,7 +110,7 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
>  		return NULL;
>  	}
>  
> -	page = vm_normal_page(vma, address, pte);
> +	page = _vm_normal_page(vma, address, pte, flags & FOLL_DEVICE);
>  	if (!page && pte_devmap(pte) && (flags & FOLL_GET)) {
>  		/*
>  		 * Only return device mapping pages in the FOLL_GET case since
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 6954c14..dea0ceb 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1690,7 +1690,7 @@ static void do_pages_stat_array(struct mm_struct *mm, unsigned long nr_pages,
>  			goto set_status;
>  
>  		/* FOLL_DUMP to ignore special (like zero) pages */
> -		page = follow_page(vma, addr, FOLL_DUMP);
> +		page = follow_page(vma, addr, FOLL_DUMP | FOLL_DEVICE);
>  
>  		err = PTR_ERR(page);
>  		if (IS_ERR(page))
> -- 
> 1.8.3.1
> 

-- 
Michal Hocko
SUSE Labs

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Re: [PATCH] mm/device-public-memory: Enable move_pages() to stat device memory

[Reza Arbab]

On Tue, Sep 26, 2017 at 01:37:07PM +0000, Michal Hocko wrote:
>On Fri 22-09-17 15:13:56, Reza Arbab wrote:
>> The move_pages() syscall can be used to find the numa node where a page
>> currently resides. This is not working for device public memory pages,
>> which erroneously report -EFAULT (unmapped or zero page).
>>
>> Enable by adding a FOLL_DEVICE flag for follow_page(), which
>> move_pages() will use. This could be done unconditionally, but adding a
>> flag seems like a safer change.
>
>I do not understand purpose of this patch. What is the numa node of a
>device memory?

Well, using hmm_devmem_pages_create() it is added to this node:

	nid = dev_to_node(device);
	if (nid < 0)
		nid = numa_mem_id();

I understand it's minimally useful information to userspace, but the 
memory does have a nid and move_pages() is supposed to be able to return 
what that is. I ran into this using a testcase which tries to verify 
that user addresses were correctly migrated to coherent device memory.

That said, I'm okay with dropping this if you don't think it's 
worthwhile.

-- 
Reza Arbab

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Re: [PATCH] mm/device-public-memory: Enable move_pages() to stat device memory

[Jerome Glisse]

On Tue, Sep 26, 2017 at 09:47:10AM -0500, Reza Arbab wrote:
> On Tue, Sep 26, 2017 at 01:37:07PM +0000, Michal Hocko wrote:
> > On Fri 22-09-17 15:13:56, Reza Arbab wrote:
> > > The move_pages() syscall can be used to find the numa node where a page
> > > currently resides. This is not working for device public memory pages,
> > > which erroneously report -EFAULT (unmapped or zero page).
> > > 
> > > Enable by adding a FOLL_DEVICE flag for follow_page(), which
> > > move_pages() will use. This could be done unconditionally, but adding a
> > > flag seems like a safer change.
> > 
> > I do not understand purpose of this patch. What is the numa node of a
> > device memory?
> 
> Well, using hmm_devmem_pages_create() it is added to this node:
> 
> 	nid = dev_to_node(device);
> 	if (nid < 0)
> 		nid = numa_mem_id();
> 
> I understand it's minimally useful information to userspace, but the memory
> does have a nid and move_pages() is supposed to be able to return what that
> is. I ran into this using a testcase which tries to verify that user
> addresses were correctly migrated to coherent device memory.
> 
> That said, I'm okay with dropping this if you don't think it's worthwhile.

Just to add a data point, PCIE devices are tie to one CPU (architecturaly PCIE
lane are connected to CPU at least on x86/ppc AFAIK) and thus to one numa node.


Right now i am traveling but i want to check that this patch does not allow
user to inadvertaly pin device memory page. I will look into it once i am
back.

Cheers,
Jerome

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Re: [PATCH] mm/device-public-memory: Enable move_pages() to stat device memory

[Michal Hocko]

On Tue 26-09-17 09:47:10, Reza Arbab wrote:
> On Tue, Sep 26, 2017 at 01:37:07PM +0000, Michal Hocko wrote:
> > On Fri 22-09-17 15:13:56, Reza Arbab wrote:
> > > The move_pages() syscall can be used to find the numa node where a page
> > > currently resides. This is not working for device public memory pages,
> > > which erroneously report -EFAULT (unmapped or zero page).
> > > 
> > > Enable by adding a FOLL_DEVICE flag for follow_page(), which
> > > move_pages() will use. This could be done unconditionally, but adding a
> > > flag seems like a safer change.
> > 
> > I do not understand purpose of this patch. What is the numa node of a
> > device memory?
> 
> Well, using hmm_devmem_pages_create() it is added to this node:
> 
> 	nid = dev_to_node(device);
> 	if (nid < 0)
> 		nid = numa_mem_id();

OK, but do all the HMM devices have concept of NUMA affinity? From the
code you are pasting they do not have to...
 
> I understand it's minimally useful information to userspace, but the memory
> does have a nid and move_pages() is supposed to be able to return what that
> is. I ran into this using a testcase which tries to verify that user
> addresses were correctly migrated to coherent device memory.
> 
> That said, I'm okay with dropping this if you don't think it's worthwhile.

I am just worried that we allow information which is not generally
sensible and I am also not sure what the userspace can actually do with
that information.
-- 
Michal Hocko
SUSE Labs

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Re: [PATCH] mm/device-public-memory: Enable move_pages() to stat device memory

[Reza Arbab]

On Tue, Sep 26, 2017 at 04:32:41PM +0000, Michal Hocko wrote:
>On Tue 26-09-17 09:47:10, Reza Arbab wrote:
>> On Tue, Sep 26, 2017 at 01:37:07PM +0000, Michal Hocko wrote:
>> > On Fri 22-09-17 15:13:56, Reza Arbab wrote:
>> > > The move_pages() syscall can be used to find the numa node where a page
>> > > currently resides. This is not working for device public memory pages,
>> > > which erroneously report -EFAULT (unmapped or zero page).
>> > >
>> > > Enable by adding a FOLL_DEVICE flag for follow_page(), which
>> > > move_pages() will use. This could be done unconditionally, but adding a
>> > > flag seems like a safer change.
>> >
>> > I do not understand purpose of this patch. What is the numa node of a
>> > device memory?
>>
>> Well, using hmm_devmem_pages_create() it is added to this node:
>>
>> 	nid = dev_to_node(device);
>> 	if (nid < 0)
>> 		nid = numa_mem_id();
>
>OK, but do all the HMM devices have concept of NUMA affinity? From the
>code you are pasting they do not have to...

I don't know the definitive answer here, but as Jerome said PCIE devices 
should, and we are heading that way with NVLink/CAPI as well. It seems 
the default is just the nearest node.

>> I understand it's minimally useful information to userspace, but the memory
>> does have a nid and move_pages() is supposed to be able to return what that
>> is. I ran into this using a testcase which tries to verify that user
>> addresses were correctly migrated to coherent device memory.
>>
>> That said, I'm okay with dropping this if you don't think it's worthwhile.
>
>I am just worried that we allow information which is not generally
>sensible and I am also not sure what the userspace can actually do with
>that information.

As mentioned, it is minimally useful, e.g. for verifying migration, so 
returning the nid seems sensible to me. Alternatively, we might at least 
change the documentation to say 

-EFAULT
    This is a zero page, a device page, or the memory area is not mapped by the process.
			 ^^^^^^^^^^^^^

-- 
Reza Arbab

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