+ mm-gup-migrate-device-coherent-pages-when-pinning-instead-of-failing.patch added to -mm tree

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/gup: migrate device coherent pages when pinning instead of failing
has been added to the -mm tree.  Its filename is
     mm-gup-migrate-device-coherent-pages-when-pinning-instead-of-failing.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/mm-gup-migrate-device-coherent-pages-when-pinning-instead-of-failing.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/mm-gup-migrate-device-coherent-pages-when-pinning-instead-of-failing.patch

Before you just go and hit "reply", please:
   a) Consider who else should be cc'ed
   b) Prefer to cc a suitable mailing list as well
   c) Ideally: find the original patch on the mailing list and do a
      reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next and is updated
there every 3-4 working days

------------------------------------------------------
From: Alistair Popple <apopple@nvidia.com>
Subject: mm/gup: 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.

[hch@lst.de: rebased to the split device memory checks, moved migrate_device_page to migrate_device.c]
Link: https://lkml.kernel.org/r/20220210072828.2930359-27-hch@lst.de
Signed-off-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Tested-by: "Sierra Guiza, Alejandro (Alex)" <alex.sierra@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Chaitanya Kulkarni <kch@nvidia.com>
Cc: Christian Knig <christian.koenig@amd.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Karol Herbst <kherbst@redhat.com>
Cc: Logan Gunthorpe <logang@deltatee.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: "Pan, Xinhui" <Xinhui.Pan@amd.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---


--- a/mm/gup.c~mm-gup-migrate-device-coherent-pages-when-pinning-instead-of-failing
+++ a/mm/gup.c
@@ -1865,9 +1865,31 @@ static long check_and_migrate_movable_pa
 			ret = -EFAULT;
 			goto unpin_pages;
 		}
+
+		/*
+		 * 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
+		 * to migrate the page out of device memory.
+		 */
 		if (is_device_coherent_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;
+				goto unpin_pages;
+			}
+			continue;
 		}
 
 		if (is_pinnable_page(head))
@@ -1907,10 +1929,13 @@ static long check_and_migrate_movable_pa
 	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;
+
+		if (gup_flags & FOLL_PIN)
+			unpin_user_page(pages[i]);
+		else
 			put_page(pages[i]);
 	}
 
--- a/mm/internal.h~mm-gup-migrate-device-coherent-pages-when-pinning-instead-of-failing
+++ a/mm/internal.h
@@ -718,5 +718,6 @@ int numa_migrate_prep(struct page *page,
 DECLARE_PER_CPU(struct per_cpu_nodestat, boot_nodestats);
 
 void free_zone_device_page(struct page *page);
+struct page *migrate_device_page(struct page *page, unsigned int gup_flags);
 
 #endif	/* __MM_INTERNAL_H */
--- a/mm/migrate_device.c~mm-gup-migrate-device-coherent-pages-when-pinning-instead-of-failing
+++ a/mm/migrate_device.c
@@ -767,3 +767,56 @@ void migrate_vma_finalize(struct migrate
 	}
 }
 EXPORT_SYMBOL(migrate_vma_finalize);
+
+/*
+ * Migrate a device coherent page back to normal memory.  The caller should have
+ * a reference on page which will be copied to the new page if migration is
+ * successful or dropped on failure.
+ */
+struct page *migrate_device_page(struct page *page, unsigned int gup_flags)
+{
+	unsigned long src_pfn, dst_pfn = 0;
+	struct migrate_vma args;
+	struct page *dpage;
+
+	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;
+}
_

Patches currently in -mm which might be from apopple@nvidia.com are

mm-remove-the-vma-check-in-migrate_vma_setup.patch
mm-gup-migrate-device-coherent-pages-when-pinning-instead-of-failing.patch