Re: [PATCH v2 2/3] mm/gup.c: Migrate device coherent pages when pinning instead of failing
On 11.02.22 00:41, Alistair Popple wrote:
> On Thursday, 10 February 2022 10:47:35 PM AEDT David Hildenbrand wrote:
>> On 10.02.22 12:39, Alistair Popple wrote:
>>> On Thursday, 10 February 2022 9:53:38 PM AEDT David Hildenbrand wrote:
On 07.02.22 05:26, Alistair Popple wrote:
> Currently any attempts to pin a device coherent page will fail. This is
> because device coherent pages need to be managed by a device driver, and
> pinning them would prevent a driver from migrating them off the device.
>
> However this is no reason to fail pinning of these pages. These are
> coherent and accessible from the CPU so can be migrated just like
> pinning ZONE_MOVABLE pages. So instead of failing all attempts to pin
> them first try migrating them out of ZONE_DEVICE.
>
> Signed-off-by: Alistair Popple
> Acked-by: Felix Kuehling
> ---
>
> Changes for v2:
>
> - Added Felix's Acked-by
> - Fixed missing check for dpage == NULL
>
> mm/gup.c | 105 ++--
> 1 file changed, 95 insertions(+), 10 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 56d9577..5e826db 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1861,6 +1861,60 @@ struct page *get_dump_page(unsigned long addr)
>
> #ifdef CONFIG_MIGRATION
> /*
> + * Migrates a device coherent page back to normal memory. Caller should
> have a
> + * reference on page which will be copied to the new page if migration is
> + * successful or dropped on failure.
> + */
> +static struct page *migrate_device_page(struct page *page,
> + unsigned int gup_flags)
> +{
> + struct page *dpage;
> + struct migrate_vma args;
> + unsigned long src_pfn, dst_pfn = 0;
> +
> + lock_page(page);
> + src_pfn = migrate_pfn(page_to_pfn(page)) | MIGRATE_PFN_MIGRATE;
> + args.src = _pfn;
> + args.dst = _pfn;
> + args.cpages = 1;
> + args.npages = 1;
> + args.vma = NULL;
> + migrate_vma_setup();
> + if (!(src_pfn & MIGRATE_PFN_MIGRATE))
> + return NULL;
> +
> + dpage = alloc_pages(GFP_USER | __GFP_NOWARN, 0);
> +
> + /*
> + * get/pin the new page now so we don't have to retry gup after
> + * migrating. We already have a reference so this should never fail.
> + */
> + if (dpage && WARN_ON_ONCE(!try_grab_page(dpage, gup_flags))) {
> + __free_pages(dpage, 0);
> + dpage = NULL;
> + }
> +
> + if (dpage) {
> + lock_page(dpage);
> + dst_pfn = migrate_pfn(page_to_pfn(dpage));
> + }
> +
> + migrate_vma_pages();
> + if (src_pfn & MIGRATE_PFN_MIGRATE)
> + copy_highpage(dpage, page);
> + migrate_vma_finalize();
> + if (dpage && !(src_pfn & MIGRATE_PFN_MIGRATE)) {
> + if (gup_flags & FOLL_PIN)
> + unpin_user_page(dpage);
> + else
> + put_page(dpage);
> + dpage = NULL;
> + }
> +
> + return dpage;
> +}
> +
> +/*
> * Check whether all pages are pinnable, if so return number of pages.
> If some
> * pages are not pinnable, migrate them, and unpin all pages. Return
> zero if
> * pages were migrated, or if some pages were not successfully isolated.
> @@ -1888,15 +1942,40 @@ static long
> check_and_migrate_movable_pages(unsigned long nr_pages,
> continue;
> prev_head = head;
> /*
> - * If we get a movable page, since we are going to be pinning
> - * these entries, try to move them out if possible.
> + * Device coherent pages are managed by a driver and should not
> + * be pinned indefinitely as it prevents the driver moving the
> + * page. So when trying to pin with FOLL_LONGTERM instead try
> + * migrating page out of device memory.
>*/
> if (is_dev_private_or_coherent_page(head)) {
> + /*
> + * device private pages will get faulted in during gup
> + * so it shouldn't be possible to see one here.
> + */
> WARN_ON_ONCE(is_device_private_page(head));
> - ret = -EFAULT;
> - goto unpin_pages;
> + WARN_ON_ONCE(PageCompound(head));
> +
> + /*
> + * migration will fail if the page is pinned, so convert
> + * the pin on the source page to a normal reference.
> + */
> + if (gup_flags & FOLL_PIN) {
> + get_page(head);
> + unpin_user_page(head);
> + }
> +
> +
Re: [PATCH v2 2/3] mm/gup.c: Migrate device coherent pages when pinning instead of failing
John Hubbard writes: > On 2/11/22 18:51, Alistair Popple wrote: […] >>> See below… >>> + } + + pages[i] = migrate_device_page(head, gup_flags); >> migrate_device_page() will return a new page that has been correctly pinned >> with gup_flags by try_grab_page(). Therefore this page can still be released >> with unpin_user_page() or put_page() as appropriate for the given gup_flags. >> The reference we had on the source page (head) always gets dropped in >> migrate_vma_finalize(). > > OK. Good. > > The above would be good to have in a comment, right around here, imho. > Because we have this marvelous mix of references for migration (get_page()) > and other, and it’s a bit hard to see that it’s all correct without a > hint or two. Ok, will do. > > … >> Which unless I’ve missed something is still the correct thing to do. >> >>> This reminds me: out of the many things to monitor, the FOLL_PIN counts >>> in /proc/vmstat are especially helpful, whenever making changes to code >>> that deals with this: >>> >>> nr_foll_pin_acquired >>> nr_foll_pin_released >>> >>> …and those should normally be equal to each other when “at rest”. >>> > > I hope this is/was run, just to be sure? Thanks for the suggestion, these remain equal to each other after running hmm-tests which confirms everything is working as expected. > > thanks,
Re: [PATCH v2 2/3] mm/gup.c: Migrate device coherent pages when pinning instead of failing
On 2/11/22 18:51, Alistair Popple wrote:
...
@@ -1888,15 +1942,40 @@ static long check_and_migrate_movable_pages(unsigned
long nr_pages,
continue;
prev_head = head;
/*
-* If we get a movable page, since we are going to be pinning
-* these entries, try to move them out if possible.
+* Device coherent pages are managed by a driver and should not
+* be pinned indefinitely as it prevents the driver moving the
+* page. So when trying to pin with FOLL_LONGTERM instead try
+* migrating page out of device memory.
*/
if (is_dev_private_or_coherent_page(head)) {
+ /*
+* device private pages will get faulted in during gup
+* so it shouldn't be possible to see one here.
+*/
WARN_ON_ONCE(is_device_private_page(head));
- ret = -EFAULT;
- goto unpin_pages;
+ WARN_ON_ONCE(PageCompound(head));
+
+ /*
+* migration will fail if the page is pinned, so convert
+* the pin on the source page to a normal reference.
+*/
+ if (gup_flags & FOLL_PIN) {
+ get_page(head);
+ unpin_user_page(head);
OK...but now gup_flags can no longer be used as a guide for how to
release these pages, right? In other words, up until this point,
FOLL_PIN meant "call unpin_user_page() in order to release". However,
now this page must be released via put_page().
This is the source page (head). We are unpinning it because we can't migrate a
pinned page, however we still need a reference on it for migrate_vma hence the
get_page followed by unpin. In the non-FOLL_PIN case we already have a
reference from gup.
See below...
+ }
+
+ pages[i] = migrate_device_page(head, gup_flags);
migrate_device_page() will return a new page that has been correctly pinned
with gup_flags by try_grab_page(). Therefore this page can still be released
with unpin_user_page() or put_page() as appropriate for the given gup_flags.
The reference we had on the source page (head) always gets dropped in
migrate_vma_finalize().
OK. Good.
The above would be good to have in a comment, right around here, imho.
Because we have this marvelous mix of references for migration (get_page())
and other, and it's a bit hard to see that it's all correct without a
hint or two.
...
Which unless I've missed something is still the correct thing to do.
This reminds me: out of the many things to monitor, the FOLL_PIN counts
in /proc/vmstat are especially helpful, whenever making changes to code
that deals with this:
nr_foll_pin_acquired
nr_foll_pin_released
...and those should normally be equal to each other when "at rest".
I hope this is/was run, just to be sure?
thanks,
--
John Hubbard
NVIDIA
Re: [PATCH v2 2/3] mm/gup.c: Migrate device coherent pages when pinning instead of failing
On Saturday, 12 February 2022 1:10:29 PM AEDT John Hubbard wrote:
> On 2/6/22 20:26, Alistair Popple wrote:
> > Currently any attempts to pin a device coherent page will fail. This is
> > because device coherent pages need to be managed by a device driver, and
> > pinning them would prevent a driver from migrating them off the device.
> >
> > However this is no reason to fail pinning of these pages. These are
> > coherent and accessible from the CPU so can be migrated just like
> > pinning ZONE_MOVABLE pages. So instead of failing all attempts to pin
> > them first try migrating them out of ZONE_DEVICE.
> >
>
> Hi Alistair and all,
>
> Here's a possible issue (below) that I really should have spotted the
> first time around, sorry for this late-breaking review. And maybe it's
> actually just my misunderstanding, anyway.
I think it might be a misunderstanding, see below.
> > Signed-off-by: Alistair Popple
> > Acked-by: Felix Kuehling
> > ---
> >
> > Changes for v2:
> >
> > - Added Felix's Acked-by
> > - Fixed missing check for dpage == NULL
> >
> > mm/gup.c | 105 ++--
> > 1 file changed, 95 insertions(+), 10 deletions(-)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 56d9577..5e826db 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -1861,6 +1861,60 @@ struct page *get_dump_page(unsigned long addr)
> >
> > #ifdef CONFIG_MIGRATION
> > /*
> > + * Migrates a device coherent page back to normal memory. Caller should
> > have a
> > + * reference on page which will be copied to the new page if migration is
> > + * successful or dropped on failure.
> > + */
> > +static struct page *migrate_device_page(struct page *page,
> > + unsigned int gup_flags)
> > +{
> > + struct page *dpage;
> > + struct migrate_vma args;
> > + unsigned long src_pfn, dst_pfn = 0;
> > +
> > + lock_page(page);
> > + src_pfn = migrate_pfn(page_to_pfn(page)) | MIGRATE_PFN_MIGRATE;
> > + args.src = _pfn;
> > + args.dst = _pfn;
> > + args.cpages = 1;
> > + args.npages = 1;
> > + args.vma = NULL;
> > + migrate_vma_setup();
> > + if (!(src_pfn & MIGRATE_PFN_MIGRATE))
> > + return NULL;
> > +
> > + dpage = alloc_pages(GFP_USER | __GFP_NOWARN, 0);
> > +
> > + /*
> > +* get/pin the new page now so we don't have to retry gup after
> > +* migrating. We already have a reference so this should never fail.
> > +*/
> > + if (dpage && WARN_ON_ONCE(!try_grab_page(dpage, gup_flags))) {
> > + __free_pages(dpage, 0);
> > + dpage = NULL;
> > + }
> > +
> > + if (dpage) {
> > + lock_page(dpage);
> > + dst_pfn = migrate_pfn(page_to_pfn(dpage));
> > + }
> > +
> > + migrate_vma_pages();
> > + if (src_pfn & MIGRATE_PFN_MIGRATE)
> > + copy_highpage(dpage, page);
> > + migrate_vma_finalize();
> > + if (dpage && !(src_pfn & MIGRATE_PFN_MIGRATE)) {
> > + if (gup_flags & FOLL_PIN)
> > + unpin_user_page(dpage);
> > + else
> > + put_page(dpage);
> > + dpage = NULL;
> > + }
> > +
> > + return dpage;
> > +}
> > +
> > +/*
> >* Check whether all pages are pinnable, if so return number of pages.
> > If some
> >* pages are not pinnable, migrate them, and unpin all pages. Return zero
> > if
> >* pages were migrated, or if some pages were not successfully isolated.
> > @@ -1888,15 +1942,40 @@ static long
> > check_and_migrate_movable_pages(unsigned long nr_pages,
> > continue;
> > prev_head = head;
> > /*
> > -* If we get a movable page, since we are going to be pinning
> > -* these entries, try to move them out if possible.
> > +* Device coherent pages are managed by a driver and should not
> > +* be pinned indefinitely as it prevents the driver moving the
> > +* page. So when trying to pin with FOLL_LONGTERM instead try
> > +* migrating page out of device memory.
> > */
> > if (is_dev_private_or_coherent_page(head)) {
> > + /*
> > +* device private pages will get faulted in during gup
> > +* so it shouldn't be possible to see one here.
> > +*/
> > WARN_ON_ONCE(is_device_private_page(head));
> > - ret = -EFAULT;
> > - goto unpin_pages;
> > + WARN_ON_ONCE(PageCompound(head));
> > +
> > + /*
> > +* migration will fail if the page is pinned, so convert
> > +* the pin on the source page to a normal reference.
> > +*/
> > + if (gup_flags & FOLL_PIN) {
> > + get_page(head);
> > + unpin_user_page(head);
>
> OK...but now gup_flags can no
Re: [PATCH v2 2/3] mm/gup.c: Migrate device coherent pages when pinning instead of failing
On 2/6/22 20:26, Alistair Popple wrote:
Currently any attempts to pin a device coherent page will fail. This is
because device coherent pages need to be managed by a device driver, and
pinning them would prevent a driver from migrating them off the device.
However this is no reason to fail pinning of these pages. These are
coherent and accessible from the CPU so can be migrated just like
pinning ZONE_MOVABLE pages. So instead of failing all attempts to pin
them first try migrating them out of ZONE_DEVICE.
Hi Alistair and all,
Here's a possible issue (below) that I really should have spotted the
first time around, sorry for this late-breaking review. And maybe it's
actually just my misunderstanding, anyway.
Signed-off-by: Alistair Popple
Acked-by: Felix Kuehling
---
Changes for v2:
- Added Felix's Acked-by
- Fixed missing check for dpage == NULL
mm/gup.c | 105 ++--
1 file changed, 95 insertions(+), 10 deletions(-)
diff --git a/mm/gup.c b/mm/gup.c
index 56d9577..5e826db 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1861,6 +1861,60 @@ struct page *get_dump_page(unsigned long addr)
#ifdef CONFIG_MIGRATION
/*
+ * Migrates a device coherent page back to normal memory. Caller should have a
+ * reference on page which will be copied to the new page if migration is
+ * successful or dropped on failure.
+ */
+static struct page *migrate_device_page(struct page *page,
+ unsigned int gup_flags)
+{
+ struct page *dpage;
+ struct migrate_vma args;
+ unsigned long src_pfn, dst_pfn = 0;
+
+ lock_page(page);
+ src_pfn = migrate_pfn(page_to_pfn(page)) | MIGRATE_PFN_MIGRATE;
+ args.src = _pfn;
+ args.dst = _pfn;
+ args.cpages = 1;
+ args.npages = 1;
+ args.vma = NULL;
+ migrate_vma_setup();
+ if (!(src_pfn & MIGRATE_PFN_MIGRATE))
+ return NULL;
+
+ dpage = alloc_pages(GFP_USER | __GFP_NOWARN, 0);
+
+ /*
+* get/pin the new page now so we don't have to retry gup after
+* migrating. We already have a reference so this should never fail.
+*/
+ if (dpage && WARN_ON_ONCE(!try_grab_page(dpage, gup_flags))) {
+ __free_pages(dpage, 0);
+ dpage = NULL;
+ }
+
+ if (dpage) {
+ lock_page(dpage);
+ dst_pfn = migrate_pfn(page_to_pfn(dpage));
+ }
+
+ migrate_vma_pages();
+ if (src_pfn & MIGRATE_PFN_MIGRATE)
+ copy_highpage(dpage, page);
+ migrate_vma_finalize();
+ if (dpage && !(src_pfn & MIGRATE_PFN_MIGRATE)) {
+ if (gup_flags & FOLL_PIN)
+ unpin_user_page(dpage);
+ else
+ put_page(dpage);
+ dpage = NULL;
+ }
+
+ return dpage;
+}
+
+/*
* Check whether all pages are pinnable, if so return number of pages. If
some
* pages are not pinnable, migrate them, and unpin all pages. Return zero if
* pages were migrated, or if some pages were not successfully isolated.
@@ -1888,15 +1942,40 @@ static long check_and_migrate_movable_pages(unsigned
long nr_pages,
continue;
prev_head = head;
/*
-* If we get a movable page, since we are going to be pinning
-* these entries, try to move them out if possible.
+* Device coherent pages are managed by a driver and should not
+* be pinned indefinitely as it prevents the driver moving the
+* page. So when trying to pin with FOLL_LONGTERM instead try
+* migrating page out of device memory.
*/
if (is_dev_private_or_coherent_page(head)) {
+ /*
+* device private pages will get faulted in during gup
+* so it shouldn't be possible to see one here.
+*/
WARN_ON_ONCE(is_device_private_page(head));
- ret = -EFAULT;
- goto unpin_pages;
+ WARN_ON_ONCE(PageCompound(head));
+
+ /*
+* migration will fail if the page is pinned, so convert
+* the pin on the source page to a normal reference.
+*/
+ if (gup_flags & FOLL_PIN) {
+ get_page(head);
+ unpin_user_page(head);
OK...but now gup_flags can no longer be used as a guide for how to
release these pages, right? In other words, up until this point,
FOLL_PIN meant "call unpin_user_page() in order to release". However,
now this page must be released via put_page().
See below...
+ }
+
+ pages[i] = migrate_device_page(head, gup_flags);
+
Re: [PATCH v2 2/3] mm/gup.c: Migrate device coherent pages when pinning instead of failing
On Thursday, 10 February 2022 10:47:35 PM AEDT David Hildenbrand wrote:
> On 10.02.22 12:39, Alistair Popple wrote:
> > On Thursday, 10 February 2022 9:53:38 PM AEDT David Hildenbrand wrote:
> >> On 07.02.22 05:26, Alistair Popple wrote:
> >>> Currently any attempts to pin a device coherent page will fail. This is
> >>> because device coherent pages need to be managed by a device driver, and
> >>> pinning them would prevent a driver from migrating them off the device.
> >>>
> >>> However this is no reason to fail pinning of these pages. These are
> >>> coherent and accessible from the CPU so can be migrated just like
> >>> pinning ZONE_MOVABLE pages. So instead of failing all attempts to pin
> >>> them first try migrating them out of ZONE_DEVICE.
> >>>
> >>> Signed-off-by: Alistair Popple
> >>> Acked-by: Felix Kuehling
> >>> ---
> >>>
> >>> Changes for v2:
> >>>
> >>> - Added Felix's Acked-by
> >>> - Fixed missing check for dpage == NULL
> >>>
> >>> mm/gup.c | 105 ++--
> >>> 1 file changed, 95 insertions(+), 10 deletions(-)
> >>>
> >>> diff --git a/mm/gup.c b/mm/gup.c
> >>> index 56d9577..5e826db 100644
> >>> --- a/mm/gup.c
> >>> +++ b/mm/gup.c
> >>> @@ -1861,6 +1861,60 @@ struct page *get_dump_page(unsigned long addr)
> >>>
> >>> #ifdef CONFIG_MIGRATION
> >>> /*
> >>> + * Migrates a device coherent page back to normal memory. Caller should
> >>> have a
> >>> + * reference on page which will be copied to the new page if migration is
> >>> + * successful or dropped on failure.
> >>> + */
> >>> +static struct page *migrate_device_page(struct page *page,
> >>> + unsigned int gup_flags)
> >>> +{
> >>> + struct page *dpage;
> >>> + struct migrate_vma args;
> >>> + unsigned long src_pfn, dst_pfn = 0;
> >>> +
> >>> + lock_page(page);
> >>> + src_pfn = migrate_pfn(page_to_pfn(page)) | MIGRATE_PFN_MIGRATE;
> >>> + args.src = _pfn;
> >>> + args.dst = _pfn;
> >>> + args.cpages = 1;
> >>> + args.npages = 1;
> >>> + args.vma = NULL;
> >>> + migrate_vma_setup();
> >>> + if (!(src_pfn & MIGRATE_PFN_MIGRATE))
> >>> + return NULL;
> >>> +
> >>> + dpage = alloc_pages(GFP_USER | __GFP_NOWARN, 0);
> >>> +
> >>> + /*
> >>> + * get/pin the new page now so we don't have to retry gup after
> >>> + * migrating. We already have a reference so this should never fail.
> >>> + */
> >>> + if (dpage && WARN_ON_ONCE(!try_grab_page(dpage, gup_flags))) {
> >>> + __free_pages(dpage, 0);
> >>> + dpage = NULL;
> >>> + }
> >>> +
> >>> + if (dpage) {
> >>> + lock_page(dpage);
> >>> + dst_pfn = migrate_pfn(page_to_pfn(dpage));
> >>> + }
> >>> +
> >>> + migrate_vma_pages();
> >>> + if (src_pfn & MIGRATE_PFN_MIGRATE)
> >>> + copy_highpage(dpage, page);
> >>> + migrate_vma_finalize();
> >>> + if (dpage && !(src_pfn & MIGRATE_PFN_MIGRATE)) {
> >>> + if (gup_flags & FOLL_PIN)
> >>> + unpin_user_page(dpage);
> >>> + else
> >>> + put_page(dpage);
> >>> + dpage = NULL;
> >>> + }
> >>> +
> >>> + return dpage;
> >>> +}
> >>> +
> >>> +/*
> >>> * Check whether all pages are pinnable, if so return number of pages.
> >>> If some
> >>> * pages are not pinnable, migrate them, and unpin all pages. Return
> >>> zero if
> >>> * pages were migrated, or if some pages were not successfully isolated.
> >>> @@ -1888,15 +1942,40 @@ static long
> >>> check_and_migrate_movable_pages(unsigned long nr_pages,
> >>> continue;
> >>> prev_head = head;
> >>> /*
> >>> - * If we get a movable page, since we are going to be pinning
> >>> - * these entries, try to move them out if possible.
> >>> + * Device coherent pages are managed by a driver and should not
> >>> + * be pinned indefinitely as it prevents the driver moving the
> >>> + * page. So when trying to pin with FOLL_LONGTERM instead try
> >>> + * migrating page out of device memory.
> >>>*/
> >>> if (is_dev_private_or_coherent_page(head)) {
> >>> + /*
> >>> + * device private pages will get faulted in during gup
> >>> + * so it shouldn't be possible to see one here.
> >>> + */
> >>> WARN_ON_ONCE(is_device_private_page(head));
> >>> - ret = -EFAULT;
> >>> - goto unpin_pages;
> >>> + WARN_ON_ONCE(PageCompound(head));
> >>> +
> >>> + /*
> >>> + * migration will fail if the page is pinned, so convert
> >>> + * the pin on the source page to a normal reference.
> >>> + */
> >>> + if (gup_flags & FOLL_PIN) {
> >>> + get_page(head);
> >>> + unpin_user_page(head);
> >>> + }
> >>> +
> >>> + pages[i] =
Re: [PATCH v2 2/3] mm/gup.c: Migrate device coherent pages when pinning instead of failing
On 10.02.22 12:39, Alistair Popple wrote:
> On Thursday, 10 February 2022 9:53:38 PM AEDT David Hildenbrand wrote:
>> On 07.02.22 05:26, Alistair Popple wrote:
>>> Currently any attempts to pin a device coherent page will fail. This is
>>> because device coherent pages need to be managed by a device driver, and
>>> pinning them would prevent a driver from migrating them off the device.
>>>
>>> However this is no reason to fail pinning of these pages. These are
>>> coherent and accessible from the CPU so can be migrated just like
>>> pinning ZONE_MOVABLE pages. So instead of failing all attempts to pin
>>> them first try migrating them out of ZONE_DEVICE.
>>>
>>> Signed-off-by: Alistair Popple
>>> Acked-by: Felix Kuehling
>>> ---
>>>
>>> Changes for v2:
>>>
>>> - Added Felix's Acked-by
>>> - Fixed missing check for dpage == NULL
>>>
>>> mm/gup.c | 105 ++--
>>> 1 file changed, 95 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/mm/gup.c b/mm/gup.c
>>> index 56d9577..5e826db 100644
>>> --- a/mm/gup.c
>>> +++ b/mm/gup.c
>>> @@ -1861,6 +1861,60 @@ struct page *get_dump_page(unsigned long addr)
>>>
>>> #ifdef CONFIG_MIGRATION
>>> /*
>>> + * Migrates a device coherent page back to normal memory. Caller should
>>> have a
>>> + * reference on page which will be copied to the new page if migration is
>>> + * successful or dropped on failure.
>>> + */
>>> +static struct page *migrate_device_page(struct page *page,
>>> + unsigned int gup_flags)
>>> +{
>>> + struct page *dpage;
>>> + struct migrate_vma args;
>>> + unsigned long src_pfn, dst_pfn = 0;
>>> +
>>> + lock_page(page);
>>> + src_pfn = migrate_pfn(page_to_pfn(page)) | MIGRATE_PFN_MIGRATE;
>>> + args.src = _pfn;
>>> + args.dst = _pfn;
>>> + args.cpages = 1;
>>> + args.npages = 1;
>>> + args.vma = NULL;
>>> + migrate_vma_setup();
>>> + if (!(src_pfn & MIGRATE_PFN_MIGRATE))
>>> + return NULL;
>>> +
>>> + dpage = alloc_pages(GFP_USER | __GFP_NOWARN, 0);
>>> +
>>> + /*
>>> +* get/pin the new page now so we don't have to retry gup after
>>> +* migrating. We already have a reference so this should never fail.
>>> +*/
>>> + if (dpage && WARN_ON_ONCE(!try_grab_page(dpage, gup_flags))) {
>>> + __free_pages(dpage, 0);
>>> + dpage = NULL;
>>> + }
>>> +
>>> + if (dpage) {
>>> + lock_page(dpage);
>>> + dst_pfn = migrate_pfn(page_to_pfn(dpage));
>>> + }
>>> +
>>> + migrate_vma_pages();
>>> + if (src_pfn & MIGRATE_PFN_MIGRATE)
>>> + copy_highpage(dpage, page);
>>> + migrate_vma_finalize();
>>> + if (dpage && !(src_pfn & MIGRATE_PFN_MIGRATE)) {
>>> + if (gup_flags & FOLL_PIN)
>>> + unpin_user_page(dpage);
>>> + else
>>> + put_page(dpage);
>>> + dpage = NULL;
>>> + }
>>> +
>>> + return dpage;
>>> +}
>>> +
>>> +/*
>>> * Check whether all pages are pinnable, if so return number of pages. If
>>> some
>>> * pages are not pinnable, migrate them, and unpin all pages. Return zero
>>> if
>>> * pages were migrated, or if some pages were not successfully isolated.
>>> @@ -1888,15 +1942,40 @@ static long
>>> check_and_migrate_movable_pages(unsigned long nr_pages,
>>> continue;
>>> prev_head = head;
>>> /*
>>> -* If we get a movable page, since we are going to be pinning
>>> -* these entries, try to move them out if possible.
>>> +* Device coherent pages are managed by a driver and should not
>>> +* be pinned indefinitely as it prevents the driver moving the
>>> +* page. So when trying to pin with FOLL_LONGTERM instead try
>>> +* migrating page out of device memory.
>>> */
>>> if (is_dev_private_or_coherent_page(head)) {
>>> + /*
>>> +* device private pages will get faulted in during gup
>>> +* so it shouldn't be possible to see one here.
>>> +*/
>>> WARN_ON_ONCE(is_device_private_page(head));
>>> - ret = -EFAULT;
>>> - goto unpin_pages;
>>> + WARN_ON_ONCE(PageCompound(head));
>>> +
>>> + /*
>>> +* migration will fail if the page is pinned, so convert
>>> +* the pin on the source page to a normal reference.
>>> +*/
>>> + if (gup_flags & FOLL_PIN) {
>>> + get_page(head);
>>> + unpin_user_page(head);
>>> + }
>>> +
>>> + pages[i] = migrate_device_page(head, gup_flags);
>>
>> For ordinary migrate_pages(), we'll unpin all pages and return 0 so the
>> caller will retry pinning by walking the page tables again.
>>
>> Why can't we apply
Re: [PATCH v2 2/3] mm/gup.c: Migrate device coherent pages when pinning instead of failing
On Thursday, 10 February 2022 9:53:38 PM AEDT David Hildenbrand wrote:
> On 07.02.22 05:26, Alistair Popple wrote:
> > Currently any attempts to pin a device coherent page will fail. This is
> > because device coherent pages need to be managed by a device driver, and
> > pinning them would prevent a driver from migrating them off the device.
> >
> > However this is no reason to fail pinning of these pages. These are
> > coherent and accessible from the CPU so can be migrated just like
> > pinning ZONE_MOVABLE pages. So instead of failing all attempts to pin
> > them first try migrating them out of ZONE_DEVICE.
> >
> > Signed-off-by: Alistair Popple
> > Acked-by: Felix Kuehling
> > ---
> >
> > Changes for v2:
> >
> > - Added Felix's Acked-by
> > - Fixed missing check for dpage == NULL
> >
> > mm/gup.c | 105 ++--
> > 1 file changed, 95 insertions(+), 10 deletions(-)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 56d9577..5e826db 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -1861,6 +1861,60 @@ struct page *get_dump_page(unsigned long addr)
> >
> > #ifdef CONFIG_MIGRATION
> > /*
> > + * Migrates a device coherent page back to normal memory. Caller should
> > have a
> > + * reference on page which will be copied to the new page if migration is
> > + * successful or dropped on failure.
> > + */
> > +static struct page *migrate_device_page(struct page *page,
> > + unsigned int gup_flags)
> > +{
> > + struct page *dpage;
> > + struct migrate_vma args;
> > + unsigned long src_pfn, dst_pfn = 0;
> > +
> > + lock_page(page);
> > + src_pfn = migrate_pfn(page_to_pfn(page)) | MIGRATE_PFN_MIGRATE;
> > + args.src = _pfn;
> > + args.dst = _pfn;
> > + args.cpages = 1;
> > + args.npages = 1;
> > + args.vma = NULL;
> > + migrate_vma_setup();
> > + if (!(src_pfn & MIGRATE_PFN_MIGRATE))
> > + return NULL;
> > +
> > + dpage = alloc_pages(GFP_USER | __GFP_NOWARN, 0);
> > +
> > + /*
> > +* get/pin the new page now so we don't have to retry gup after
> > +* migrating. We already have a reference so this should never fail.
> > +*/
> > + if (dpage && WARN_ON_ONCE(!try_grab_page(dpage, gup_flags))) {
> > + __free_pages(dpage, 0);
> > + dpage = NULL;
> > + }
> > +
> > + if (dpage) {
> > + lock_page(dpage);
> > + dst_pfn = migrate_pfn(page_to_pfn(dpage));
> > + }
> > +
> > + migrate_vma_pages();
> > + if (src_pfn & MIGRATE_PFN_MIGRATE)
> > + copy_highpage(dpage, page);
> > + migrate_vma_finalize();
> > + if (dpage && !(src_pfn & MIGRATE_PFN_MIGRATE)) {
> > + if (gup_flags & FOLL_PIN)
> > + unpin_user_page(dpage);
> > + else
> > + put_page(dpage);
> > + dpage = NULL;
> > + }
> > +
> > + return dpage;
> > +}
> > +
> > +/*
> > * Check whether all pages are pinnable, if so return number of pages. If
> > some
> > * pages are not pinnable, migrate them, and unpin all pages. Return zero
> > if
> > * pages were migrated, or if some pages were not successfully isolated.
> > @@ -1888,15 +1942,40 @@ static long
> > check_and_migrate_movable_pages(unsigned long nr_pages,
> > continue;
> > prev_head = head;
> > /*
> > -* If we get a movable page, since we are going to be pinning
> > -* these entries, try to move them out if possible.
> > +* Device coherent pages are managed by a driver and should not
> > +* be pinned indefinitely as it prevents the driver moving the
> > +* page. So when trying to pin with FOLL_LONGTERM instead try
> > +* migrating page out of device memory.
> > */
> > if (is_dev_private_or_coherent_page(head)) {
> > + /*
> > +* device private pages will get faulted in during gup
> > +* so it shouldn't be possible to see one here.
> > +*/
> > WARN_ON_ONCE(is_device_private_page(head));
> > - ret = -EFAULT;
> > - goto unpin_pages;
> > + WARN_ON_ONCE(PageCompound(head));
> > +
> > + /*
> > +* migration will fail if the page is pinned, so convert
> > +* the pin on the source page to a normal reference.
> > +*/
> > + if (gup_flags & FOLL_PIN) {
> > + get_page(head);
> > + unpin_user_page(head);
> > + }
> > +
> > + pages[i] = migrate_device_page(head, gup_flags);
>
> For ordinary migrate_pages(), we'll unpin all pages and return 0 so the
> caller will retry pinning by walking the page tables again.
>
> Why can't we apply the same mechanism here? This "let's avoid
Re: [PATCH v2 2/3] mm/gup.c: Migrate device coherent pages when pinning instead of failing
On 07.02.22 05:26, Alistair Popple wrote:
> Currently any attempts to pin a device coherent page will fail. This is
> because device coherent pages need to be managed by a device driver, and
> pinning them would prevent a driver from migrating them off the device.
>
> However this is no reason to fail pinning of these pages. These are
> coherent and accessible from the CPU so can be migrated just like
> pinning ZONE_MOVABLE pages. So instead of failing all attempts to pin
> them first try migrating them out of ZONE_DEVICE.
>
> Signed-off-by: Alistair Popple
> Acked-by: Felix Kuehling
> ---
>
> Changes for v2:
>
> - Added Felix's Acked-by
> - Fixed missing check for dpage == NULL
>
> mm/gup.c | 105 ++--
> 1 file changed, 95 insertions(+), 10 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 56d9577..5e826db 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1861,6 +1861,60 @@ struct page *get_dump_page(unsigned long addr)
>
> #ifdef CONFIG_MIGRATION
> /*
> + * Migrates a device coherent page back to normal memory. Caller should have
> a
> + * reference on page which will be copied to the new page if migration is
> + * successful or dropped on failure.
> + */
> +static struct page *migrate_device_page(struct page *page,
> + unsigned int gup_flags)
> +{
> + struct page *dpage;
> + struct migrate_vma args;
> + unsigned long src_pfn, dst_pfn = 0;
> +
> + lock_page(page);
> + src_pfn = migrate_pfn(page_to_pfn(page)) | MIGRATE_PFN_MIGRATE;
> + args.src = _pfn;
> + args.dst = _pfn;
> + args.cpages = 1;
> + args.npages = 1;
> + args.vma = NULL;
> + migrate_vma_setup();
> + if (!(src_pfn & MIGRATE_PFN_MIGRATE))
> + return NULL;
> +
> + dpage = alloc_pages(GFP_USER | __GFP_NOWARN, 0);
> +
> + /*
> + * get/pin the new page now so we don't have to retry gup after
> + * migrating. We already have a reference so this should never fail.
> + */
> + if (dpage && WARN_ON_ONCE(!try_grab_page(dpage, gup_flags))) {
> + __free_pages(dpage, 0);
> + dpage = NULL;
> + }
> +
> + if (dpage) {
> + lock_page(dpage);
> + dst_pfn = migrate_pfn(page_to_pfn(dpage));
> + }
> +
> + migrate_vma_pages();
> + if (src_pfn & MIGRATE_PFN_MIGRATE)
> + copy_highpage(dpage, page);
> + migrate_vma_finalize();
> + if (dpage && !(src_pfn & MIGRATE_PFN_MIGRATE)) {
> + if (gup_flags & FOLL_PIN)
> + unpin_user_page(dpage);
> + else
> + put_page(dpage);
> + dpage = NULL;
> + }
> +
> + return dpage;
> +}
> +
> +/*
> * Check whether all pages are pinnable, if so return number of pages. If
> some
> * pages are not pinnable, migrate them, and unpin all pages. Return zero if
> * pages were migrated, or if some pages were not successfully isolated.
> @@ -1888,15 +1942,40 @@ static long check_and_migrate_movable_pages(unsigned
> long nr_pages,
> continue;
> prev_head = head;
> /*
> - * If we get a movable page, since we are going to be pinning
> - * these entries, try to move them out if possible.
> + * Device coherent pages are managed by a driver and should not
> + * be pinned indefinitely as it prevents the driver moving the
> + * page. So when trying to pin with FOLL_LONGTERM instead try
> + * migrating page out of device memory.
>*/
> if (is_dev_private_or_coherent_page(head)) {
> + /*
> + * device private pages will get faulted in during gup
> + * so it shouldn't be possible to see one here.
> + */
> WARN_ON_ONCE(is_device_private_page(head));
> - ret = -EFAULT;
> - goto unpin_pages;
> + WARN_ON_ONCE(PageCompound(head));
> +
> + /*
> + * migration will fail if the page is pinned, so convert
> + * the pin on the source page to a normal reference.
> + */
> + if (gup_flags & FOLL_PIN) {
> + get_page(head);
> + unpin_user_page(head);
> + }
> +
> + pages[i] = migrate_device_page(head, gup_flags);
For ordinary migrate_pages(), we'll unpin all pages and return 0 so the
caller will retry pinning by walking the page tables again.
Why can't we apply the same mechanism here? This "let's avoid another
walk" looks unnecessary complicated to me, but I might be wrong.
--
Thanks,
David / dhildenb
[PATCH v2 2/3] mm/gup.c: Migrate device coherent pages when pinning instead of failing
Currently any attempts to pin a device coherent page will fail. This is
because device coherent pages need to be managed by a device driver, and
pinning them would prevent a driver from migrating them off the device.
However this is no reason to fail pinning of these pages. These are
coherent and accessible from the CPU so can be migrated just like
pinning ZONE_MOVABLE pages. So instead of failing all attempts to pin
them first try migrating them out of ZONE_DEVICE.
Signed-off-by: Alistair Popple
Acked-by: Felix Kuehling
---
Changes for v2:
- Added Felix's Acked-by
- Fixed missing check for dpage == NULL
mm/gup.c | 105 ++--
1 file changed, 95 insertions(+), 10 deletions(-)
diff --git a/mm/gup.c b/mm/gup.c
index 56d9577..5e826db 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1861,6 +1861,60 @@ struct page *get_dump_page(unsigned long addr)
#ifdef CONFIG_MIGRATION
/*
+ * Migrates a device coherent page back to normal memory. Caller should have a
+ * reference on page which will be copied to the new page if migration is
+ * successful or dropped on failure.
+ */
+static struct page *migrate_device_page(struct page *page,
+ unsigned int gup_flags)
+{
+ struct page *dpage;
+ struct migrate_vma args;
+ unsigned long src_pfn, dst_pfn = 0;
+
+ lock_page(page);
+ src_pfn = migrate_pfn(page_to_pfn(page)) | MIGRATE_PFN_MIGRATE;
+ args.src = _pfn;
+ args.dst = _pfn;
+ args.cpages = 1;
+ args.npages = 1;
+ args.vma = NULL;
+ migrate_vma_setup();
+ if (!(src_pfn & MIGRATE_PFN_MIGRATE))
+ return NULL;
+
+ dpage = alloc_pages(GFP_USER | __GFP_NOWARN, 0);
+
+ /*
+* get/pin the new page now so we don't have to retry gup after
+* migrating. We already have a reference so this should never fail.
+*/
+ if (dpage && WARN_ON_ONCE(!try_grab_page(dpage, gup_flags))) {
+ __free_pages(dpage, 0);
+ dpage = NULL;
+ }
+
+ if (dpage) {
+ lock_page(dpage);
+ dst_pfn = migrate_pfn(page_to_pfn(dpage));
+ }
+
+ migrate_vma_pages();
+ if (src_pfn & MIGRATE_PFN_MIGRATE)
+ copy_highpage(dpage, page);
+ migrate_vma_finalize();
+ if (dpage && !(src_pfn & MIGRATE_PFN_MIGRATE)) {
+ if (gup_flags & FOLL_PIN)
+ unpin_user_page(dpage);
+ else
+ put_page(dpage);
+ dpage = NULL;
+ }
+
+ return dpage;
+}
+
+/*
* Check whether all pages are pinnable, if so return number of pages. If some
* pages are not pinnable, migrate them, and unpin all pages. Return zero if
* pages were migrated, or if some pages were not successfully isolated.
@@ -1888,15 +1942,40 @@ static long check_and_migrate_movable_pages(unsigned
long nr_pages,
continue;
prev_head = head;
/*
-* If we get a movable page, since we are going to be pinning
-* these entries, try to move them out if possible.
+* Device coherent pages are managed by a driver and should not
+* be pinned indefinitely as it prevents the driver moving the
+* page. So when trying to pin with FOLL_LONGTERM instead try
+* migrating page out of device memory.
*/
if (is_dev_private_or_coherent_page(head)) {
+ /*
+* device private pages will get faulted in during gup
+* so it shouldn't be possible to see one here.
+*/
WARN_ON_ONCE(is_device_private_page(head));
- ret = -EFAULT;
- goto unpin_pages;
+ WARN_ON_ONCE(PageCompound(head));
+
+ /*
+* migration will fail if the page is pinned, so convert
+* the pin on the source page to a normal reference.
+*/
+ if (gup_flags & FOLL_PIN) {
+ get_page(head);
+ unpin_user_page(head);
+ }
+
+ pages[i] = migrate_device_page(head, gup_flags);
+ if (!pages[i]) {
+ ret = -EBUSY;
+ break;
+ }
+ continue;
}
+ /*
+* If we get a movable page, since we are going to be pinning
+* these entries, try to move them out if possible.
+*/
if (!is_pinnable_page(head)) {
if (PageHuge(head)) {
if
