Re: [PATCH 2/3] mm/gup.c: Migrate device coherent pages when pinning instead of failing

[Alistair Popple <apopple@nvidia.com>]

On Wednesday, 2 February 2022 2:03:01 AM AEDT Felix Kuehling wrote:
> 
> Am 2022-02-01 um 02:05 schrieb Alistair Popple:
> > 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 <apopple@nvidia.com>
> 
> Thank you for working on this. I have two questions inline.
> 
> Other than that, patches 1 and 2 are
> 
> Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
> 
> 
> > ---
> >   mm/gup.c | 105 ++++++++++++++++++++++++++++++++++++++++++++++++++------
> >   1 file changed, 95 insertions(+), 10 deletions(-)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index f596b93..2cbef54 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -1834,6 +1834,60 @@ 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 = &src_pfn;
> > +	args.dst = &dst_pfn;
> > +	args.cpages = 1;
> > +	args.npages = 1;
> > +	args.vma = NULL;
> > +	migrate_vma_setup(&args);
> > +	if (!(src_pfn & MIGRATE_PFN_MIGRATE))
> > +		return NULL;
> > +
> > +	dpage = alloc_pages(GFP_USER | __GFP_NOWARN, 0);
> 
> Don't you need to check dpage for NULL before the try_grab_page call below?

Yes, thanks for pointing that out. Will fix for v2.

> > +
> > +	/*
> > +	 * 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 (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(&args);
> > +	if (src_pfn & MIGRATE_PFN_MIGRATE)
> > +		copy_highpage(dpage, page);
> 
> Can't dpage can be NULL here as well?

No - migrate_vma_pages() will clear src_pfn & MIGRATE_PFN_MIGRATE if no
destination page is provided in dst_pfn.

> Regards,
>    Felix
> 
> 
> > +	migrate_vma_finalize(&args);
> > +	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.
> > @@ -1861,15 +1915,40 @@ 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 (!isolate_huge_page(head, &movable_page_list))
> > @@ -1897,16 +1976,22 @@ static long check_and_migrate_movable_pages(unsigned long nr_pages,
> >   	 * If list is empty, and no isolation errors, means that all pages are
> >   	 * in the correct zone.
> >   	 */
> > -	if (list_empty(&movable_page_list) && !isolation_error_count)
> > +	if (!ret && list_empty(&movable_page_list) && !isolation_error_count)
> >   		return nr_pages;
> >   
> > -unpin_pages:
> > -	if (gup_flags & FOLL_PIN) {
> > -		unpin_user_pages(pages, nr_pages);
> > -	} else {
> > -		for (i = 0; i < nr_pages; i++)
> > +	for (i = 0; i < nr_pages; i++)
> > +		if (!pages[i])
> > +			continue;
> > +		else if (gup_flags & FOLL_PIN)
> > +			unpin_user_page(pages[i]);
> > +		else
> >   			put_page(pages[i]);
> > +
> > +	if (ret && !list_empty(&movable_page_list)) {
> > +		putback_movable_pages(&movable_page_list);
> > +		return ret;
> >   	}
> > +
> >   	if (!list_empty(&movable_page_list)) {
> >   		ret = migrate_pages(&movable_page_list, alloc_migration_target,
> >   				    NULL, (unsigned long)&mtc, MIGRATE_SYNC,
>