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

[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 mm-unstable branch.  Its filename is
     mm-gup-migrate-device-coherent-pages-when-pinning-instead-of-failing.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-gup-migrate-device-coherent-pages-when-pinning-instead-of-failing.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Alistair Popple <apopple@nvidia.com>
Subject: mm/gup: migrate device coherent pages when pinning instead of failing
Date: Tue, 31 May 2022 15:00:33 -0500

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/20220531200041.24904-6-alex.sierra@amd.com
Signed-off-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/gup.c            |   47 +++++++++++++++++++++++++++++++++----
 mm/internal.h       |    1 
 mm/migrate_device.c |   53 ++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 96 insertions(+), 5 deletions(-)

--- a/mm/gup.c~mm-gup-migrate-device-coherent-pages-when-pinning-instead-of-failing
+++ a/mm/gup.c
@@ -1927,9 +1927,43 @@ static long check_and_migrate_movable_pa
 			continue;
 		prev_folio = folio;
 
-		if (folio_is_pinnable(folio))
+		/*
+		 * Device private pages will get faulted in during gup so it
+		 * shouldn't be possible to see one here.
+		 */
+		if (WARN_ON_ONCE(folio_is_device_private(folio))) {
+			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 (folio_is_device_coherent(folio)) {
+			WARN_ON_ONCE(PageCompound(&folio->page));
+
+			/*
+			 * 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(&folio->page);
+				unpin_user_page(&folio->page);
+			}
+
+			pages[i] = migrate_device_page(&folio->page, gup_flags);
+			if (!pages[i]) {
+				ret = -EBUSY;
+				goto unpin_pages;
+			}
 			continue;
+		}
 
+		if (folio_is_pinnable(folio))
+			continue;
 		/*
 		 * Try to move out any movable page before pinning the range.
 		 */
@@ -1965,10 +1999,13 @@ 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
@@ -853,6 +853,7 @@ int numa_migrate_prep(struct page *page,
 		      unsigned long addr, int page_nid, int *flags);
 
 void free_zone_device_page(struct page *page);
+struct page *migrate_device_page(struct page *page, unsigned int gup_flags);
 
 /*
  * mm/gup.c
--- a/mm/migrate_device.c~mm-gup-migrate-device-coherent-pages-when-pinning-instead-of-failing
+++ a/mm/migrate_device.c
@@ -794,3 +794,56 @@ 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