The goal for this series is to avoid device private memory TLB
invalidations when migrating a range of addresses from system
memory to device private memory and some of those pages have already
been migrated. The approach taken is to introduce a new mmu notifier
invalidation event type and use that in the device driver to skip
invalidation callbacks from migrate_vma_setup(). The device driver is
also then expected to handle device MMU invalidations as part of the
migrate_vma_setup(), migrate_vma_pages(), migrate_vma_finalize() process.
Note that this is opt-in. A device driver can simply invalidate its MMU
in the mmu notifier callback and not handle MMU invalidations in the
migration sequence.
This series is based on Jason Gunthorpe's HMM tree (linux-5.8.0-rc4).
Also, this replaces the need for the following two patches I sent:
("mm: fix migrate_vma_setup() src_owner and normal pages")
https://lore.kernel.org/linux-mm/[email protected]
("nouveau: fix mixed normal and device private page migration")
https://lore.kernel.org/lkml/[email protected]
Changes in v4:
Added reviewed-by from Bharata B Rao.
Removed dead code checking for source device private page in lib/test_hmm.c
dmirror_migrate_alloc_and_copy() since the source filter flag guarantees
that.
Added patch 6 to remove a redundant invalidation in migrate_vma_pages().
Changes in v3:
Changed the direction field "dir" to a "flags" field and renamed
src_owner to pgmap_owner.
Fixed a locking issue in nouveau for the migration invalidation.
Added a HMM selftest test case to exercise the HMM test driver
invalidation changes.
Removed reviewed-by Bharata B Rao since this version is moderately
changed.
Changes in v2:
Rebase to Jason Gunthorpe's HMM tree.
Added reviewed-by from Bharata B Rao.
Rename the mmu_notifier_range::data field to migrate_pgmap_owner as
suggested by Jason Gunthorpe.
Ralph Campbell (6):
nouveau: fix storing invalid ptes
mm/migrate: add a flags parameter to migrate_vma
mm/notifier: add migration invalidation type
nouveau/svm: use the new migration invalidation
mm/hmm/test: use the new migration invalidation
mm/migrate: remove range invalidation in migrate_vma_pages()
arch/powerpc/kvm/book3s_hv_uvmem.c | 4 +-
drivers/gpu/drm/nouveau/nouveau_dmem.c | 19 ++++++--
drivers/gpu/drm/nouveau/nouveau_svm.c | 21 ++++-----
drivers/gpu/drm/nouveau/nouveau_svm.h | 13 +++++-
.../drm/nouveau/nvkm/subdev/mmu/vmmgp100.c | 13 ++++--
include/linux/migrate.h | 16 +++++--
include/linux/mmu_notifier.h | 7 +++
lib/test_hmm.c | 43 +++++++++----------
mm/migrate.c | 34 +++++----------
tools/testing/selftests/vm/hmm-tests.c | 18 ++++++--
10 files changed, 112 insertions(+), 76 deletions(-)
--
2.20.1
When migrating the special zero page, migrate_vma_pages() calls
mmu_notifier_invalidate_range_start() before replacing the zero page
PFN in the CPU page tables. This is unnecessary since the range was
invalidated in migrate_vma_setup() and the page table entry is checked
to be sure it hasn't changed between migrate_vma_setup() and
migrate_vma_pages(). Therefore, remove the redundant invalidation.
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ZHtBRMp/Zu/ZLGRxP6lBqZo4wHMHbuW3fXOvPCrYTD5YsCCLv+Ao4RmreWyec2wBTk
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n9jm36j4kRTUg
Signed-off-by: Ralph Campbell <[email protected]>
---
mm/migrate.c | 20 --------------------
1 file changed, 20 deletions(-)
diff --git a/mm/migrate.c b/mm/migrate.c
index 96e1f41a991e..36076ba2f51a 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2877,9 +2877,7 @@ void migrate_vma_pages(struct migrate_vma *migrate)
{
const unsigned long npages = migrate->npages;
const unsigned long start = migrate->start;
- struct mmu_notifier_range range;
unsigned long addr, i;
- bool notified = false;
for (i = 0, addr = start; i < npages; addr += PAGE_SIZE, i++) {
struct page *newpage = migrate_pfn_to_page(migrate->dst[i]);
@@ -2895,16 +2893,6 @@ void migrate_vma_pages(struct migrate_vma *migrate)
if (!page) {
if (!(migrate->src[i] & MIGRATE_PFN_MIGRATE))
continue;
- if (!notified) {
- notified = true;
-
- mmu_notifier_range_init(&range,
- MMU_NOTIFY_CLEAR, 0,
- NULL,
- migrate->vma->vm_mm,
- addr, migrate->end);
- mmu_notifier_invalidate_range_start(&range);
- }
migrate_vma_insert_page(migrate, addr, newpage,
&migrate->src[i],
&migrate->dst[i]);
@@ -2937,14 +2925,6 @@ void migrate_vma_pages(struct migrate_vma *migrate)
if (r != MIGRATEPAGE_SUCCESS)
migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
}
-
- /*
- * No need to double call mmu_notifier->invalidate_range() callback as
- * the above ptep_clear_flush_notify() inside migrate_vma_insert_page()
- * did already call it.
- */
- if (notified)
- mmu_notifier_invalidate_range_only_end(&range);
}
EXPORT_SYMBOL(migrate_vma_pages);
--
2.20.1
Use the new MMU_NOTIFY_MIGRATE event to skip GPU MMU invalidations of
device private memory and handle the invalidation in the driver as part
of migrating device private memory.
Signed-off-by: Ralph Campbell <[email protected]>
---
drivers/gpu/drm/nouveau/nouveau_dmem.c | 15 ++++++++++++---
drivers/gpu/drm/nouveau/nouveau_svm.c | 21 +++++++++------------
drivers/gpu/drm/nouveau/nouveau_svm.h | 13 ++++++++++++-
3 files changed, 33 insertions(+), 16 deletions(-)
diff --git a/drivers/gpu/drm/nouveau/nouveau_dmem.c b/drivers/gpu/drm/nouveau/nouveau_dmem.c
index 78b9e3c2a5b3..511fe04cd680 100644
--- a/drivers/gpu/drm/nouveau/nouveau_dmem.c
+++ b/drivers/gpu/drm/nouveau/nouveau_dmem.c
@@ -140,6 +140,7 @@ static vm_fault_t nouveau_dmem_fault_copy_one(struct nouveau_drm *drm,
{
struct device *dev = drm->dev->dev;
struct page *dpage, *spage;
+ struct nouveau_svmm *svmm;
spage = migrate_pfn_to_page(args->src[0]);
if (!spage || !(args->src[0] & MIGRATE_PFN_MIGRATE))
@@ -154,14 +155,19 @@ static vm_fault_t nouveau_dmem_fault_copy_one(struct nouveau_drm *drm,
if (dma_mapping_error(dev, *dma_addr))
goto error_free_page;
+ svmm = spage->zone_device_data;
+ mutex_lock(&svmm->mutex);
+ nouveau_svmm_invalidate(svmm, args->start, args->end);
if (drm->dmem->migrate.copy_func(drm, 1, NOUVEAU_APER_HOST, *dma_addr,
NOUVEAU_APER_VRAM, nouveau_dmem_page_addr(spage)))
goto error_dma_unmap;
+ mutex_unlock(&svmm->mutex);
args->dst[0] = migrate_pfn(page_to_pfn(dpage)) | MIGRATE_PFN_LOCKED;
return 0;
error_dma_unmap:
+ mutex_unlock(&svmm->mutex);
dma_unmap_page(dev, *dma_addr, PAGE_SIZE, DMA_BIDIRECTIONAL);
error_free_page:
__free_page(dpage);
@@ -531,7 +537,8 @@ nouveau_dmem_init(struct nouveau_drm *drm)
}
static unsigned long nouveau_dmem_migrate_copy_one(struct nouveau_drm *drm,
- unsigned long src, dma_addr_t *dma_addr, u64 *pfn)
+ struct nouveau_svmm *svmm, unsigned long src,
+ dma_addr_t *dma_addr, u64 *pfn)
{
struct device *dev = drm->dev->dev;
struct page *dpage, *spage;
@@ -561,6 +568,7 @@ static unsigned long nouveau_dmem_migrate_copy_one(struct nouveau_drm *drm,
goto out_free_page;
}
+ dpage->zone_device_data = svmm;
*pfn = NVIF_VMM_PFNMAP_V0_V | NVIF_VMM_PFNMAP_V0_VRAM |
((paddr >> PAGE_SHIFT) << NVIF_VMM_PFNMAP_V0_ADDR_SHIFT);
if (src & MIGRATE_PFN_WRITE)
@@ -584,8 +592,8 @@ static void nouveau_dmem_migrate_chunk(struct nouveau_drm *drm,
unsigned long addr = args->start, nr_dma = 0, i;
for (i = 0; addr < args->end; i++) {
- args->dst[i] = nouveau_dmem_migrate_copy_one(drm, args->src[i],
- dma_addrs + nr_dma, pfns + i);
+ args->dst[i] = nouveau_dmem_migrate_copy_one(drm, svmm,
+ args->src[i], dma_addrs + nr_dma, pfns + i);
if (!dma_mapping_error(drm->dev->dev, dma_addrs[nr_dma]))
nr_dma++;
addr += PAGE_SIZE;
@@ -616,6 +624,7 @@ nouveau_dmem_migrate_vma(struct nouveau_drm *drm,
struct migrate_vma args = {
.vma = vma,
.start = start,
+ .pgmap_owner = drm->dev,
.flags = MIGRATE_VMA_SELECT_SYSTEM,
};
unsigned long i;
diff --git a/drivers/gpu/drm/nouveau/nouveau_svm.c b/drivers/gpu/drm/nouveau/nouveau_svm.c
index c5f8ca6fb2e3..67b068ac57b2 100644
--- a/drivers/gpu/drm/nouveau/nouveau_svm.c
+++ b/drivers/gpu/drm/nouveau/nouveau_svm.c
@@ -93,17 +93,6 @@ nouveau_ivmm_find(struct nouveau_svm *svm, u64 inst)
return NULL;
}
-struct nouveau_svmm {
- struct mmu_notifier notifier;
- struct nouveau_vmm *vmm;
- struct {
- unsigned long start;
- unsigned long limit;
- } unmanaged;
-
- struct mutex mutex;
-};
-
#define SVMM_DBG(s,f,a...) \
NV_DEBUG((s)->vmm->cli->drm, "svm-%p: "f"\n", (s), ##a)
#define SVMM_ERR(s,f,a...) \
@@ -246,7 +235,7 @@ nouveau_svmm_join(struct nouveau_svmm *svmm, u64 inst)
}
/* Invalidate SVMM address-range on GPU. */
-static void
+void
nouveau_svmm_invalidate(struct nouveau_svmm *svmm, u64 start, u64 limit)
{
if (limit > start) {
@@ -279,6 +268,14 @@ nouveau_svmm_invalidate_range_start(struct mmu_notifier *mn,
if (unlikely(!svmm->vmm))
goto out;
+ /*
+ * Ignore invalidation callbacks for device private pages since
+ * the invalidation is handled as part of the migration process.
+ */
+ if (update->event == MMU_NOTIFY_MIGRATE &&
+ update->migrate_pgmap_owner == svmm->vmm->cli->drm->dev)
+ goto out;
+
if (limit > svmm->unmanaged.start && start < svmm->unmanaged.limit) {
if (start < svmm->unmanaged.start) {
nouveau_svmm_invalidate(svmm, start,
diff --git a/drivers/gpu/drm/nouveau/nouveau_svm.h b/drivers/gpu/drm/nouveau/nouveau_svm.h
index f0fcd1b72e8b..e7d63d7f0c2d 100644
--- a/drivers/gpu/drm/nouveau/nouveau_svm.h
+++ b/drivers/gpu/drm/nouveau/nouveau_svm.h
@@ -1,11 +1,21 @@
#ifndef __NOUVEAU_SVM_H__
#define __NOUVEAU_SVM_H__
#include <nvif/os.h>
+#include <linux/mmu_notifier.h>
struct drm_device;
struct drm_file;
struct nouveau_drm;
-struct nouveau_svmm;
+struct nouveau_svmm {
+ struct mmu_notifier notifier;
+ struct nouveau_vmm *vmm;
+ struct {
+ unsigned long start;
+ unsigned long limit;
+ } unmanaged;
+
+ struct mutex mutex;
+};
#if IS_ENABLED(CONFIG_DRM_NOUVEAU_SVM)
void nouveau_svm_init(struct nouveau_drm *);
@@ -19,6 +29,7 @@ int nouveau_svmm_join(struct nouveau_svmm *, u64 inst);
void nouveau_svmm_part(struct nouveau_svmm *, u64 inst);
int nouveau_svmm_bind(struct drm_device *, void *, struct drm_file *);
+void nouveau_svmm_invalidate(struct nouveau_svmm *svmm, u64 start, u64 limit);
u64 *nouveau_pfns_alloc(unsigned long npages);
void nouveau_pfns_free(u64 *pfns);
void nouveau_pfns_map(struct nouveau_svmm *svmm, struct mm_struct *mm,
--
2.20.1
On Thu, Jul 23, 2020 at 03:30:04PM -0700, Ralph Campbell wrote:
> When migrating the special zero page, migrate_vma_pages() calls
> mmu_notifier_invalidate_range_start() before replacing the zero page
> PFN in the CPU page tables. This is unnecessary since the range was
> invalidated in migrate_vma_setup() and the page table entry is checked
> to be sure it hasn't changed between migrate_vma_setup() and
> migrate_vma_pages(). Therefore, remove the redundant invalidation.
I don't follow this logic, the purpose of the invalidation is also to
clear out anything that may be mirroring this VA, and "the page hasn't
changed" doesn't seem to rule out that case?
I'm also not sure I follow where the zero page came from?
Jason
On Thu, Jul 23, 2020 at 03:29:58PM -0700, Ralph Campbell wrote:
> The goal for this series is to avoid device private memory TLB
> invalidations when migrating a range of addresses from system
> memory to device private memory and some of those pages have already
> been migrated. The approach taken is to introduce a new mmu notifier
> invalidation event type and use that in the device driver to skip
> invalidation callbacks from migrate_vma_setup(). The device driver is
> also then expected to handle device MMU invalidations as part of the
> migrate_vma_setup(), migrate_vma_pages(), migrate_vma_finalize() process.
> Note that this is opt-in. A device driver can simply invalidate its MMU
> in the mmu notifier callback and not handle MMU invalidations in the
> migration sequence.
>
> This series is based on Jason Gunthorpe's HMM tree (linux-5.8.0-rc4).
>
> Also, this replaces the need for the following two patches I sent:
> ("mm: fix migrate_vma_setup() src_owner and normal pages")
> https://lore.kernel.org/linux-mm/[email protected]
> ("nouveau: fix mixed normal and device private page migration")
> https://lore.kernel.org/lkml/[email protected]
>
> Changes in v4:
> Added reviewed-by from Bharata B Rao.
> Removed dead code checking for source device private page in lib/test_hmm.c
> dmirror_migrate_alloc_and_copy() since the source filter flag guarantees
> that.
> Added patch 6 to remove a redundant invalidation in migrate_vma_pages().
>
> Changes in v3:
> Changed the direction field "dir" to a "flags" field and renamed
> src_owner to pgmap_owner.
> Fixed a locking issue in nouveau for the migration invalidation.
> Added a HMM selftest test case to exercise the HMM test driver
> invalidation changes.
> Removed reviewed-by Bharata B Rao since this version is moderately
> changed.
>
> Changes in v2:
> Rebase to Jason Gunthorpe's HMM tree.
> Added reviewed-by from Bharata B Rao.
> Rename the mmu_notifier_range::data field to migrate_pgmap_owner as
> suggested by Jason Gunthorpe.
>
> Ralph Campbell (6):
> nouveau: fix storing invalid ptes
> mm/migrate: add a flags parameter to migrate_vma
> mm/notifier: add migration invalidation type
> nouveau/svm: use the new migration invalidation
> mm/hmm/test: use the new migration invalidation
Applied to the hmm tree with the modification I noted, I think all the
comments in the past versions were addressed. I will accumulate more
Reviews if any come.
> mm/migrate: remove range invalidation in migrate_vma_pages()
Let's have some discussion on this new patch please, at least I don't
follow it yet.
Thanks,
Jason
On 7/28/20 12:19 PM, Jason Gunthorpe wrote:
> On Thu, Jul 23, 2020 at 03:30:04PM -0700, Ralph Campbell wrote:
>> When migrating the special zero page, migrate_vma_pages() calls
>> mmu_notifier_invalidate_range_start() before replacing the zero page
>> PFN in the CPU page tables. This is unnecessary since the range was
>> invalidated in migrate_vma_setup() and the page table entry is checked
>> to be sure it hasn't changed between migrate_vma_setup() and
>> migrate_vma_pages(). Therefore, remove the redundant invalidation.
>
> I don't follow this logic, the purpose of the invalidation is also to
> clear out anything that may be mirroring this VA, and "the page hasn't
> changed" doesn't seem to rule out that case?
>
> I'm also not sure I follow where the zero page came from?
The zero page comes from an anonymous private VMA that is read-only
and the user level CPU process tries to read the page data (or any
other read page fault).
> Jason
>
The overall migration process is:
mmap_read_lock()
migrate_vma_setup()
// invalidates range, locks/isolates pages, puts migration entry in page table
<driver allocates destination pages and copies source to dest>
migrate_vma_pages()
// moves source struct page info to destination struct page info.
// clears migration flag for pages that can't be migrated.
<driver updates device page tables for pages still migrating, rollback pages not migrating>
migrate_vma_finalize()
// replaces migration page table entry with destination page PFN.
mmap_read_unlock()
Since the address range is invalidated in the migrate_vma_setup() stage,
and the page is isolated from the LRU cache, locked, unmapped, and the page table
holds a migration entry (so the page can't be faulted and the CPU page table set
valid again), and there are no extra page references (pins), the page
"should not be modified".
For pte_none()/is_zero_pfn() entries, migrate_vma_setup() leaves the
pte_none()/is_zero_pfn() entry in place but does still call
mmu_notifier_invalidate_range_start() for the whole range being migrated.
In the migrate_vma_pages() step, the pte page table is locked and the
pte entry checked to be sure it is still pte_none/is_zero_pfn(). If not,
the new page isn't inserted. If it is still none/zero, the new device private
struct page is inserted into the page table, replacing the pte_none()/is_zero_pfn()
page table entry. The secondary MMUs were already invalidated in the migrate_vma_setup()
step and a pte_none() or zero page can't be modified so the only invalidation needed
is the CPU TLB(s) for clearing the special zero page PTE entry.
Two devices could both try to do the migrate_vma_*() sequence and proceed in parallel up
to the migrate_vma_pages() step and try to install a new page for the hole/zero PTE but
only one will win and the other fail.
On Tue, Jul 28, 2020 at 03:04:07PM -0700, Ralph Campbell wrote:
>
> On 7/28/20 12:19 PM, Jason Gunthorpe wrote:
> > On Thu, Jul 23, 2020 at 03:30:04PM -0700, Ralph Campbell wrote:
> > > When migrating the special zero page, migrate_vma_pages() calls
> > > mmu_notifier_invalidate_range_start() before replacing the zero page
> > > PFN in the CPU page tables. This is unnecessary since the range was
> > > invalidated in migrate_vma_setup() and the page table entry is checked
> > > to be sure it hasn't changed between migrate_vma_setup() and
> > > migrate_vma_pages(). Therefore, remove the redundant invalidation.
> >
> > I don't follow this logic, the purpose of the invalidation is also to
> > clear out anything that may be mirroring this VA, and "the page hasn't
> > changed" doesn't seem to rule out that case?
> >
> > I'm also not sure I follow where the zero page came from?
>
> The zero page comes from an anonymous private VMA that is read-only
> and the user level CPU process tries to read the page data (or any
> other read page fault).
>
> > Jason
> >
>
> The overall migration process is:
>
> mmap_read_lock()
>
> migrate_vma_setup()
> // invalidates range, locks/isolates pages, puts migration entry in page table
>
> <driver allocates destination pages and copies source to dest>
>
> migrate_vma_pages()
> // moves source struct page info to destination struct page info.
> // clears migration flag for pages that can't be migrated.
>
> <driver updates device page tables for pages still migrating, rollback pages not migrating>
>
> migrate_vma_finalize()
> // replaces migration page table entry with destination page PFN.
>
> mmap_read_unlock()
>
> Since the address range is invalidated in the migrate_vma_setup() stage,
> and the page is isolated from the LRU cache, locked, unmapped, and the page table
> holds a migration entry (so the page can't be faulted and the CPU page table set
> valid again), and there are no extra page references (pins), the page
> "should not be modified".
That is the physical page though, it doesn't prove nobody else is
reading the PTE.
> For pte_none()/is_zero_pfn() entries, migrate_vma_setup() leaves the
> pte_none()/is_zero_pfn() entry in place but does still call
> mmu_notifier_invalidate_range_start() for the whole range being migrated.
Ok..
> In the migrate_vma_pages() step, the pte page table is locked and the
> pte entry checked to be sure it is still pte_none/is_zero_pfn(). If not,
> the new page isn't inserted. If it is still none/zero, the new device private
> struct page is inserted into the page table, replacing the pte_none()/is_zero_pfn()
> page table entry. The secondary MMUs were already invalidated in the migrate_vma_setup()
> step and a pte_none() or zero page can't be modified so the only invalidation needed
> is the CPU TLB(s) for clearing the special zero page PTE entry.
No, the secondary MMU was invalidated but the invalidation start/end
range was exited. That means a secondary MMU is immeidately able to
reload the zero page into its MMU cache.
When this code replaces the PTE that has a zero page it also has to
invalidate again so that secondary MMU's are guaranteed to pick up the
new PTE value.
So, I still don't understand how this is safe?
Jason
On 7/31/20 12:15 PM, Jason Gunthorpe wrote:
> On Tue, Jul 28, 2020 at 03:04:07PM -0700, Ralph Campbell wrote:
>>
>> On 7/28/20 12:19 PM, Jason Gunthorpe wrote:
>>> On Thu, Jul 23, 2020 at 03:30:04PM -0700, Ralph Campbell wrote:
>>>> When migrating the special zero page, migrate_vma_pages() calls
>>>> mmu_notifier_invalidate_range_start() before replacing the zero page
>>>> PFN in the CPU page tables. This is unnecessary since the range was
>>>> invalidated in migrate_vma_setup() and the page table entry is checked
>>>> to be sure it hasn't changed between migrate_vma_setup() and
>>>> migrate_vma_pages(). Therefore, remove the redundant invalidation.
>>>
>>> I don't follow this logic, the purpose of the invalidation is also to
>>> clear out anything that may be mirroring this VA, and "the page hasn't
>>> changed" doesn't seem to rule out that case?
>>>
>>> I'm also not sure I follow where the zero page came from?
>>
>> The zero page comes from an anonymous private VMA that is read-only
>> and the user level CPU process tries to read the page data (or any
>> other read page fault).
>>
>>> Jason
>>>
>>
>> The overall migration process is:
>>
>> mmap_read_lock()
>>
>> migrate_vma_setup()
>> // invalidates range, locks/isolates pages, puts migration entry in page table
>>
>> <driver allocates destination pages and copies source to dest>
>>
>> migrate_vma_pages()
>> // moves source struct page info to destination struct page info.
>> // clears migration flag for pages that can't be migrated.
>>
>> <driver updates device page tables for pages still migrating, rollback pages not migrating>
>>
>> migrate_vma_finalize()
>> // replaces migration page table entry with destination page PFN.
>>
>> mmap_read_unlock()
>>
>> Since the address range is invalidated in the migrate_vma_setup() stage,
>> and the page is isolated from the LRU cache, locked, unmapped, and the page table
>> holds a migration entry (so the page can't be faulted and the CPU page table set
>> valid again), and there are no extra page references (pins), the page
>> "should not be modified".
>
> That is the physical page though, it doesn't prove nobody else is
> reading the PTE.
>
>> For pte_none()/is_zero_pfn() entries, migrate_vma_setup() leaves the
>> pte_none()/is_zero_pfn() entry in place but does still call
>> mmu_notifier_invalidate_range_start() for the whole range being migrated.
>
> Ok..
>
>> In the migrate_vma_pages() step, the pte page table is locked and the
>> pte entry checked to be sure it is still pte_none/is_zero_pfn(). If not,
>> the new page isn't inserted. If it is still none/zero, the new device private
>> struct page is inserted into the page table, replacing the pte_none()/is_zero_pfn()
>> page table entry. The secondary MMUs were already invalidated in the migrate_vma_setup()
>> step and a pte_none() or zero page can't be modified so the only invalidation needed
>> is the CPU TLB(s) for clearing the special zero page PTE entry.
>
> No, the secondary MMU was invalidated but the invalidation start/end
> range was exited. That means a secondary MMU is immeidately able to
> reload the zero page into its MMU cache.
>
> When this code replaces the PTE that has a zero page it also has to
> invalidate again so that secondary MMU's are guaranteed to pick up the
> new PTE value.
>
> So, I still don't understand how this is safe?
>
> Jason
Oops, you are right of course. I was only thinking of the device doing the migration
and forgetting about a second device faulting on the same page.
You can drop patch from the series.