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[HMM v13 07/18] mm/ZONE_DEVICE/x86: add support for un-addressable device memory

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  [HMM v13 07/18] mm/ZONE_DEVICE/x86: add support for un-addressable device memory Jérôme Glisse <jglisse@redhat.com> - 2016-11-18 18:20 +0100
    Re: [HMM v13 07/18] mm/ZONE_DEVICE/x86: add support for  un-addressable device memory Balbir Singh <bsingharora@gmail.com> - 2016-11-21 03:10 +0100
      Re: [HMM v13 07/18] mm/ZONE_DEVICE/x86: add support for  un-addressable device memory Jerome Glisse <jglisse@redhat.com> - 2016-11-21 06:10 +0100

#1525558 — [HMM v13 07/18] mm/ZONE_DEVICE/x86: add support for un-addressable device memory

It does not need much, just skip populating kernel linear mapping
for range of un-addressable device memory (it is pick so that there
is no physical memory resource overlapping it). All the logic is in
share mm code.

Only support x86-64 as this feature doesn't make much sense with
constrained virtual address space of 32bits architecture.

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
---
 arch/x86/mm/init_64.c | 28 ++++++++++++++--------------
 1 file changed, 14 insertions(+), 14 deletions(-)

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 8c4abb0..556f7bb 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -661,13 +661,17 @@ int arch_add_memory(int nid, u64 start, u64 size, int flags)
 	unsigned long nr_pages = size >> PAGE_SHIFT;
 	int ret;
 
-	/* Need to add support for device and unaddressable memory if needed */
-	if (flags & MEMORY_UNADDRESSABLE) {
-		BUG();
-		return -EINVAL;
-	}
-
-	init_memory_mapping(start, start + size);
+	/*
+	 * We get un-addressable memory when some one is adding a ZONE_DEVICE
+	 * to have struct page for a device memory which is not accessible by
+	 * the CPU so it is pointless to have a linear kernel mapping of such
+	 * memory.
+	 *
+	 * Core mm should make sure it never set a pte pointing to such fake
+	 * physical range.
+	 */
+	if (!(flags & MEMORY_UNADDRESSABLE))
+		init_memory_mapping(start, start + size);
 
 	ret = __add_pages(nid, zone, start_pfn, nr_pages);
 	WARN_ON_ONCE(ret);
@@ -972,12 +976,6 @@ int __ref arch_remove_memory(u64 start, u64 size, int flags)
 	struct zone *zone;
 	int ret;
 
-	/* Need to add support for device and unaddressable memory if needed */
-	if (flags & MEMORY_UNADDRESSABLE) {
-		BUG();
-		return -EINVAL;
-	}
-
 	/* With altmap the first mapped page is offset from @start */
 	altmap = to_vmem_altmap((unsigned long) page);
 	if (altmap)
@@ -985,7 +983,9 @@ int __ref arch_remove_memory(u64 start, u64 size, int flags)
 	zone = page_zone(page);
 	ret = __remove_pages(zone, start_pfn, nr_pages);
 	WARN_ON_ONCE(ret);
-	kernel_physical_mapping_remove(start, start + size);
+
+	if (!(flags & MEMORY_UNADDRESSABLE))
+		kernel_physical_mapping_remove(start, start + size);
 
 	return ret;
 }
-- 
2.4.3

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#1526330 — Re: [HMM v13 07/18] mm/ZONE_DEVICE/x86: add support for un-addressable device memory

On 19/11/16 05:18, Jérôme Glisse wrote:
> It does not need much, just skip populating kernel linear mapping
> for range of un-addressable device memory (it is pick so that there
> is no physical memory resource overlapping it). All the logic is in
> share mm code.
> 
> Only support x86-64 as this feature doesn't make much sense with
> constrained virtual address space of 32bits architecture.
> 

Is there a reason this would not work on powerpc64 for example?
Could you document the limitations -- testing/APIs/missing features?

Balbir Singh.

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#1526362 — Re: [HMM v13 07/18] mm/ZONE_DEVICE/x86: add support for un-addressable device memory

On Mon, Nov 21, 2016 at 01:08:56PM +1100, Balbir Singh wrote:
> 
> 
> On 19/11/16 05:18, Jérôme Glisse wrote:
> > It does not need much, just skip populating kernel linear mapping
> > for range of un-addressable device memory (it is pick so that there
> > is no physical memory resource overlapping it). All the logic is in
> > share mm code.
> > 
> > Only support x86-64 as this feature doesn't make much sense with
> > constrained virtual address space of 32bits architecture.
> > 
> 
> Is there a reason this would not work on powerpc64 for example?
> Could you document the limitations -- testing/APIs/missing features?

It should be straight forward for powerpc64, i haven't done it but i
certainly can try to get access to some powerpc64 and add support for
it.

The only thing to do is to avoid creating kernel linear mapping for the
un-addressable memory (just for safety reasons we do not want any read/
write to invalid physical address).

Cheers,
Jérôme

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