[PATCH v2 4/4] mm/gup: adapt get_user_page_vma_remote() to never return NULL

[Lorenzo Stoakes <lstoakes@gmail.com>]

get_user_pages_remote() will never return 0 except in the case of
FOLL_NOWAIT being specified, which we explicitly disallow.

This simplifies error handling for the caller and avoids the awkwardness of
dealing with both errors and failing to pin. Failing to pin here is an
error.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
---
 arch/arm64/kernel/mte.c |  4 ++--
 include/linux/mm.h      | 12 +++++++++---
 kernel/events/uprobes.c |  4 ++--
 mm/memory.c             |  3 +--
 4 files changed, 14 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 4edecaac8f91..8878b392df58 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -411,8 +411,8 @@ static int __access_remote_tags(struct mm_struct *mm, unsigned long addr,
 		struct page *page = get_user_page_vma_remote(mm, addr,
 							     gup_flags, &vma);
 
-		if (IS_ERR_OR_NULL(page)) {
-			err = page == NULL ? -EIO : PTR_ERR(page);
+		if (IS_ERR(page)) {
+			err = PTR_ERR(page);
 			break;
 		}
 
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7b89f7bd420d..fa608cba041f 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2425,6 +2425,9 @@ long pin_user_pages_remote(struct mm_struct *mm,
 			   unsigned int gup_flags, struct page **pages,
 			   int *locked);
 
+/*
+ * Retrieves a single page alongside its VMA. Does not support FOLL_NOWAIT.
+ */
 static inline struct page *get_user_page_vma_remote(struct mm_struct *mm,
 						    unsigned long addr,
 						    int gup_flags,
@@ -2432,12 +2435,15 @@ static inline struct page *get_user_page_vma_remote(struct mm_struct *mm,
 {
 	struct page *page;
 	struct vm_area_struct *vma;
-	int got = get_user_pages_remote(mm, addr, 1, gup_flags, &page, NULL);
+	int got;
+
+	if (WARN_ON_ONCE(unlikely(gup_flags & FOLL_NOWAIT)))
+		return ERR_PTR(-EINVAL);
+
+	got = get_user_pages_remote(mm, addr, 1, gup_flags, &page, NULL);
 
 	if (got < 0)
 		return ERR_PTR(got);
-	if (got == 0)
-		return NULL;
 
 	vma = vma_lookup(mm, addr);
 	if (WARN_ON_ONCE(!vma)) {
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 3048589e2e85..435aac1d8c27 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -474,8 +474,8 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
 		gup_flags |= FOLL_SPLIT_PMD;
 	/* Read the page with vaddr into memory */
 	old_page = get_user_page_vma_remote(mm, vaddr, gup_flags, &vma);
-	if (IS_ERR_OR_NULL(old_page))
-		return old_page ? PTR_ERR(old_page) : 0;
+	if (IS_ERR(old_page))
+		return PTR_ERR(old_page);
 
 	ret = verify_opcode(old_page, vaddr, &opcode);
 	if (ret <= 0)
diff --git a/mm/memory.c b/mm/memory.c
index e2743aa95b56..f2eef3d1cf58 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5905,7 +5905,7 @@ static int __access_remote_vm(struct mm_struct *mm, unsigned long addr,
 		struct page *page = get_user_page_vma_remote(mm, addr,
 							     gup_flags, &vma);
 
-		if (IS_ERR_OR_NULL(page)) {
+		if (IS_ERR(page)) {
 			/* We might need to expand the stack to access it */
 			vma = vma_lookup(mm, addr);
 			if (!vma) {
@@ -5919,7 +5919,6 @@ static int __access_remote_vm(struct mm_struct *mm, unsigned long addr,
 				continue;
 			}
 
-
 			/*
 			 * Check if this is a VM_IO | VM_PFNMAP VMA, which
 			 * we can access using slightly different code.
-- 
2.42.0