mm/rmap: convert make_device_exclusive_range() to make_device_exclusive()

The single "real" user in the tree of make_device_exclusive_range() always
requests making only a single address exclusive.  The current
implementation is hard to fix for properly supporting anonymous THP /
large folios and for avoiding messing with rmap walks in weird ways.

So let's always process a single address/page and return folio + page to
minimize page -> folio lookups.  This is a preparation for further
changes.

Reject any non-anonymous or hugetlb folios early, directly after GUP.

While at it, extend the documentation of make_device_exclusive() to
clarify some things.

Link: https://lkml.kernel.org/r/20250210193801.781278-4-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Simona Vetter <simona.vetter@ffwll.ch>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Tested-by: Alistair Popple <apopple@nvidia.com>
Cc: Alex Shi <alexs@kernel.org>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: Dave Airlie <airlied@gmail.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Karol Herbst <kherbst@redhat.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Lyude <lyude@redhat.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: SeongJae Park <sj@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yanteng Si <si.yanteng@linux.dev>
Cc: Barry Song <v-songbaohua@oppo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 61 insertions, 42 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index 17fbfa61f7ef..7ccf850565d3 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -2495,70 +2495,89 @@ static bool folio_make_device_exclusive(struct folio *folio,
 		.arg = &args,
 	};
 
-	/*
-	 * Restrict to anonymous folios for now to avoid potential writeback
-	 * issues.
-	 */
-	if (!folio_test_anon(folio) || folio_test_hugetlb(folio))
-		return false;
-
 	rmap_walk(folio, &rwc);
 
 	return args.valid && !folio_mapcount(folio);
 }
 
 /**
- * make_device_exclusive_range() - Mark a range for exclusive use by a device
+ * make_device_exclusive() - Mark a page for exclusive use by a device
  * @mm: mm_struct of associated target process
- * @start: start of the region to mark for exclusive device access
- * @end: end address of region
- * @pages: returns the pages which were successfully marked for exclusive access
+ * @addr: the virtual address to mark for exclusive device access
  * @owner: passed to MMU_NOTIFY_EXCLUSIVE range notifier to allow filtering
+ * @foliop: folio pointer will be stored here on success.
+ *
+ * This function looks up the page mapped at the given address, grabs a
+ * folio reference, locks the folio and replaces the PTE with special
+ * device-exclusive PFN swap entry, preventing access through the process
+ * page tables. The function will return with the folio locked and referenced.
  *
- * Returns: number of pages found in the range by GUP. A page is marked for
- * exclusive access only if the page pointer is non-NULL.
+ * On fault, the device-exclusive entries are replaced with the original PTE
+ * under folio lock, after calling MMU notifiers.
  *
- * This function finds ptes mapping page(s) to the given address range, locks
- * them and replaces mappings with special swap entries preventing userspace CPU
- * access. On fault these entries are replaced with the original mapping after
- * calling MMU notifiers.
+ * Only anonymous non-hugetlb folios are supported and the VMA must have
+ * write permissions such that we can fault in the anonymous page writable
+ * in order to mark it exclusive. The caller must hold the mmap_lock in read
+ * mode.
  *
  * A driver using this to program access from a device must use a mmu notifier
  * critical section to hold a device specific lock during programming. Once
- * programming is complete it should drop the page lock and reference after
+ * programming is complete it should drop the folio lock and reference after
  * which point CPU access to the page will revoke the exclusive access.
+ *
+ * Notes:
+ *   #. This function always operates on individual PTEs mapping individual
+ *      pages. PMD-sized THPs are first remapped to be mapped by PTEs before
+ *      the conversion happens on a single PTE corresponding to @addr.
+ *   #. While concurrent access through the process page tables is prevented,
+ *      concurrent access through other page references (e.g., earlier GUP
+ *      invocation) is not handled and not supported.
+ *   #. device-exclusive entries are considered "clean" and "old" by core-mm.
+ *      Device drivers must update the folio state when informed by MMU
+ *      notifiers.
+ *
+ * Returns: pointer to mapped page on success, otherwise a negative error.
  */
-int make_device_exclusive_range(struct mm_struct *mm, unsigned long start,
-				unsigned long end, struct page **pages,
-				void *owner)
+struct page *make_device_exclusive(struct mm_struct *mm, unsigned long addr,
+		void *owner, struct folio **foliop)
 {
-	long npages = (end - start) >> PAGE_SHIFT;
-	long i;
+	struct folio *folio;
+	struct page *page;
+	long npages;
+
+	mmap_assert_locked(mm);
 
-	npages = get_user_pages_remote(mm, start, npages,
+	/*
+	 * Fault in the page writable and try to lock it; note that if the
+	 * address would already be marked for exclusive use by a device,
+	 * the GUP call would undo that first by triggering a fault.
+	 */
+	npages = get_user_pages_remote(mm, addr, 1,
 				       FOLL_GET | FOLL_WRITE | FOLL_SPLIT_PMD,
-				       pages, NULL);
-	if (npages < 0)
-		return npages;
-
-	for (i = 0; i < npages; i++, start += PAGE_SIZE) {
-		struct folio *folio = page_folio(pages[i]);
-		if (PageTail(pages[i]) || !folio_trylock(folio)) {
-			folio_put(folio);
-			pages[i] = NULL;
-			continue;
-		}
+				       &page, NULL);
+	if (npages != 1)
+		return ERR_PTR(npages);
+	folio = page_folio(page);
 
-		if (!folio_make_device_exclusive(folio, mm, start, owner)) {
-			folio_unlock(folio);
-			folio_put(folio);
-			pages[i] = NULL;
-		}
+	if (!folio_test_anon(folio) || folio_test_hugetlb(folio)) {
+		folio_put(folio);
+		return ERR_PTR(-EOPNOTSUPP);
+	}
+
+	if (!folio_trylock(folio)) {
+		folio_put(folio);
+		return ERR_PTR(-EBUSY);
 	}
 
-	return npages;
+	if (!folio_make_device_exclusive(folio, mm, addr, owner)) {
+		folio_unlock(folio);
+		folio_put(folio);
+		return ERR_PTR(-EBUSY);
+	}
+	*foliop = folio;
+	return page;
 }
-EXPORT_SYMBOL_GPL(make_device_exclusive_range);
+EXPORT_SYMBOL_GPL(make_device_exclusive);
 #endif
 
 void __put_anon_vma(struct anon_vma *anon_vma)