diff mbox series

[v8,06/15] mm: remove the vma check in migrate_vma_setup()

Message ID 20220707190349.9778-7-alex.sierra@amd.com (mailing list archive)
State Deferred, archived
Headers show
Series Add MEMORY_DEVICE_COHERENT for coherent device memory mapping | expand

Commit Message

Sierra Guiza, Alejandro (Alex) July 7, 2022, 7:03 p.m. UTC 
From: Alistair Popple <apopple@nvidia.com>

migrate_vma_setup() checks that a valid vma is passed so that the page
tables can be walked to find the pfns associated with a given address
range. However in some cases the pfns are already known, such as when
migrating device coherent pages during pin_user_pages() meaning a valid
vma isn't required.

Signed-off-by: Alistair Popple <apopple@nvidia.com>
Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 mm/migrate_device.c | 34 +++++++++++++++++-----------------
 1 file changed, 17 insertions(+), 17 deletions(-)

Comments

David Hildenbrand July 11, 2022, 1:52 p.m. UTC | #1 
On 07.07.22 21:03, Alex Sierra wrote:
> From: Alistair Popple <apopple@nvidia.com>
> 
> migrate_vma_setup() checks that a valid vma is passed so that the page
> tables can be walked to find the pfns associated with a given address
> range. However in some cases the pfns are already known, such as when
> migrating device coherent pages during pin_user_pages() meaning a valid
> vma isn't required.

As raised in my other reply, without a VMA ... it feels odd to use a
"migrate_vma" API. For an internal (mm/migrate_device.c) use case it is
ok I guess, but it certainly adds a bit of confusion. For example,
because migrate_vma_setup() will undo ref+lock not obtained by it.

I guess the interesting point is that

a) Besides migrate_vma_pages() and migrate_vma_setup(), the ->vma is unused.

b) migrate_vma_setup() does collect+unmap+cleanup if unmap failed.

c) With our source page in our hands, we cannot be processing a hole in
a VMA.



Not sure if it's better. but I would

a) Enforce in migrate_vma_setup() that there is a VMA. Code outside of
mm/migrate_device.c shouldn't be doing some hacks like this.

b) Don't call migrate_vma_setup() from migrate_device_page(), but
directly migrate_vma_unmap() and add a comment.


That will leave a single change to this patch (migrate_vma_pages()). But
is that even required? Because ....

> @@ -685,7 +685,7 @@ void migrate_vma_pages(struct migrate_vma *migrate)
>  			continue;
>  		}
>  
> -		if (!page) {
> +		if (!page && migrate->vma) {

How could we ever have !page in case of migrate_device_page()?

Instead, I think a VM_BUG_ON(migrate->vma); should hold and you can just
simplify.

>  			if (!(migrate->src[i] & MIGRATE_PFN_MIGRATE))
>  				continue;
>  			if (!notified) {
Alistair Popple July 14, 2022, 5:31 a.m. UTC | #2 
David Hildenbrand <david@redhat.com> writes:

> On 07.07.22 21:03, Alex Sierra wrote:
>> From: Alistair Popple <apopple@nvidia.com>
>>
>> migrate_vma_setup() checks that a valid vma is passed so that the page
>> tables can be walked to find the pfns associated with a given address
>> range. However in some cases the pfns are already known, such as when
>> migrating device coherent pages during pin_user_pages() meaning a valid
>> vma isn't required.
>
> As raised in my other reply, without a VMA ... it feels odd to use a
> "migrate_vma" API. For an internal (mm/migrate_device.c) use case it is
> ok I guess, but it certainly adds a bit of confusion. For example,
> because migrate_vma_setup() will undo ref+lock not obtained by it.
>
> I guess the interesting point is that
>
> a) Besides migrate_vma_pages() and migrate_vma_setup(), the ->vma is unused.
>
> b) migrate_vma_setup() does collect+unmap+cleanup if unmap failed.
>
> c) With our source page in our hands, we cannot be processing a hole in
> a VMA.
>
>
>
> Not sure if it's better. but I would
>
> a) Enforce in migrate_vma_setup() that there is a VMA. Code outside of
> mm/migrate_device.c shouldn't be doing some hacks like this.
>
> b) Don't call migrate_vma_setup() from migrate_device_page(), but
> directly migrate_vma_unmap() and add a comment.
>
>
> That will leave a single change to this patch (migrate_vma_pages()). But
> is that even required? Because ....
>
>> @@ -685,7 +685,7 @@ void migrate_vma_pages(struct migrate_vma *migrate)
>>  			continue;
>>  		}
>>
>> -		if (!page) {
>> +		if (!page && migrate->vma) {
>
> How could we ever have !page in case of migrate_device_page()?

Oh good point. This patch was originally part of a larger series I was
working on at the time but you're right - for migrate_device_page() we
should never hit this case. I will respin the next patch (number 7 in
this series) to include this.

> Instead, I think a VM_BUG_ON(migrate->vma); should hold and you can just
> simplify.
>
>>  			if (!(migrate->src[i] & MIGRATE_PFN_MIGRATE))
>>  				continue;
>>  			if (!notified) {
diff mbox series

Patch

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 18bc6483f63a..cf9668376c5a 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -486,24 +486,24 @@  int migrate_vma_setup(struct migrate_vma *args)
 
 	args->start &= PAGE_MASK;
 	args->end &= PAGE_MASK;
-	if (!args->vma || is_vm_hugetlb_page(args->vma) ||
-	    (args->vma->vm_flags & VM_SPECIAL) || vma_is_dax(args->vma))
-		return -EINVAL;
-	if (nr_pages <= 0)
-		return -EINVAL;
-	if (args->start < args->vma->vm_start ||
-	    args->start >= args->vma->vm_end)
-		return -EINVAL;
-	if (args->end <= args->vma->vm_start || args->end > args->vma->vm_end)
-		return -EINVAL;
 	if (!args->src || !args->dst)
 		return -EINVAL;
-
-	memset(args->src, 0, sizeof(*args->src) * nr_pages);
-	args->cpages = 0;
-	args->npages = 0;
-
-	migrate_vma_collect(args);
+	if (args->vma) {
+		if (is_vm_hugetlb_page(args->vma) ||
+		    (args->vma->vm_flags & VM_SPECIAL) || vma_is_dax(args->vma))
+			return -EINVAL;
+		if (args->start < args->vma->vm_start ||
+		    args->start >= args->vma->vm_end)
+			return -EINVAL;
+		if (args->end <= args->vma->vm_start ||
+		    args->end > args->vma->vm_end)
+			return -EINVAL;
+		memset(args->src, 0, sizeof(*args->src) * nr_pages);
+		args->cpages = 0;
+		args->npages = 0;
+
+		migrate_vma_collect(args);
+	}
 
 	if (args->cpages)
 		migrate_vma_unmap(args);
@@ -685,7 +685,7 @@  void migrate_vma_pages(struct migrate_vma *migrate)
 			continue;
 		}
 
-		if (!page) {
+		if (!page && migrate->vma) {
 			if (!(migrate->src[i] & MIGRATE_PFN_MIGRATE))
 				continue;
 			if (!notified) {