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From:   Alistair Popple <apopple@nvidia.com>
To:     <akpm@linux-foundation.org>, <linux-mm@kvack.org>,
        John Hubbard <jhubbard@nvidia.com>
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Subject: Re: [PATCH v2 2/3] mm/gup.c: Migrate device coherent pages when pinning instead of failing
Date:   Sat, 12 Feb 2022 13:51:15 +1100
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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 <apopple@nvidia.com>
> > Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
> > ---
> > 
> > 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 = &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);
> > +
> > +	/*
> > +	 * 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(&args);
> > +	if (src_pfn & MIGRATE_PFN_MIGRATE)
> > +		copy_highpage(dpage, page);
> > +	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.
> > @@ -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().

> > +			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))
> > @@ -1924,16 +2003,22 @@ 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]);
> 
> ...and here, for example, we are still trusting gup_flags to decide how
> to release the pages.

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".
> 
> 
> thanks,
> > +		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,
> 
>