[PATCH 2/9] userfaultfd/shmem: combine shmem_{mcopy_atomic,mfill_zeropage}_pte

[Axel Rasmussen <axelrasmussen@google.com>]

Previously, we did a dance where we had one calling path in
userfaultfd.c (mfill_atomic_pte), but then we split it into two in
shmem_fs.h (shmem_{mcopy_atomic,mfill_zeropage}_pte), and then rejoined
into a single shared function in shmem.c (shmem_mfill_atomic_pte).

This is all a bit overly complex. Just call the single combined shmem
function directly, allowing us to clean up various branches,
boilerplate, etc.

While we're touching this function, two other small cleanup changes:
- offset is equivalent to pgoff, so we can get rid of offset entirely.
- Split two VM_BUG_ON cases into two statements. This means the line
  number reported when the BUG is hit specifies exactly which condition
  was true.

Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
---
 include/linux/shmem_fs.h | 15 +++++-------
 mm/shmem.c               | 52 +++++++++++++---------------------------
 mm/userfaultfd.c         | 10 +++-----
 3 files changed, 25 insertions(+), 52 deletions(-)

diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index d82b6f396588..919e36671fe6 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -122,21 +122,18 @@ static inline bool shmem_file(struct file *file)
 extern bool shmem_charge(struct inode *inode, long pages);
 extern void shmem_uncharge(struct inode *inode, long pages);
 
+#ifdef CONFIG_USERFAULTFD
 #ifdef CONFIG_SHMEM
 extern int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
 				  struct vm_area_struct *dst_vma,
 				  unsigned long dst_addr,
 				  unsigned long src_addr,
+				  bool zeropage,
 				  struct page **pagep);
-extern int shmem_mfill_zeropage_pte(struct mm_struct *dst_mm,
-				    pmd_t *dst_pmd,
-				    struct vm_area_struct *dst_vma,
-				    unsigned long dst_addr);
-#else
+#else /* !CONFIG_SHMEM */
 #define shmem_mcopy_atomic_pte(dst_mm, dst_pte, dst_vma, dst_addr, \
-			       src_addr, pagep)        ({ BUG(); 0; })
-#define shmem_mfill_zeropage_pte(dst_mm, dst_pmd, dst_vma, \
-				 dst_addr)      ({ BUG(); 0; })
-#endif
+			       src_addr, zeropage, pagep)       ({ BUG(); 0; })
+#endif /* CONFIG_SHMEM */
+#endif /* CONFIG_USERFAULTFD */
 
 #endif
diff --git a/mm/shmem.c b/mm/shmem.c
index b2db4ed0fbc7..c21f20cc4204 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2354,13 +2354,14 @@ static struct inode *shmem_get_inode(struct super_block *sb, const struct inode
 	return inode;
 }
 
-static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
-				  pmd_t *dst_pmd,
-				  struct vm_area_struct *dst_vma,
-				  unsigned long dst_addr,
-				  unsigned long src_addr,
-				  bool zeropage,
-				  struct page **pagep)
+#ifdef CONFIG_USERFAULTFD
+int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm,
+			   pmd_t *dst_pmd,
+			   struct vm_area_struct *dst_vma,
+			   unsigned long dst_addr,
+			   unsigned long src_addr,
+			   bool zeropage,
+			   struct page **pagep)
 {
 	struct inode *inode = file_inode(dst_vma->vm_file);
 	struct shmem_inode_info *info = SHMEM_I(inode);
@@ -2372,7 +2373,7 @@ static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
 	struct page *page;
 	pte_t _dst_pte, *dst_pte;
 	int ret;
-	pgoff_t offset, max_off;
+	pgoff_t max_off;
 
 	ret = -ENOMEM;
 	if (!shmem_inode_acct_block(inode, 1))
@@ -2383,7 +2384,7 @@ static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
 		if (!page)
 			goto out_unacct_blocks;
 
-		if (!zeropage) {	/* mcopy_atomic */
+		if (!zeropage) {	/* COPY */
 			page_kaddr = kmap_atomic(page);
 			ret = copy_from_user(page_kaddr,
 					     (const void __user *)src_addr,
@@ -2397,7 +2398,7 @@ static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
 				/* don't free the page */
 				return -ENOENT;
 			}
-		} else {		/* mfill_zeropage_atomic */
+		} else {		/* ZEROPAGE */
 			clear_highpage(page);
 		}
 	} else {
@@ -2405,15 +2406,15 @@ static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
 		*pagep = NULL;
 	}
 
-	VM_BUG_ON(PageLocked(page) || PageSwapBacked(page));
+	VM_BUG_ON(PageLocked(page));
+	VM_BUG_ON(PageSwapBacked(page));
 	__SetPageLocked(page);
 	__SetPageSwapBacked(page);
 	__SetPageUptodate(page);
 
 	ret = -EFAULT;
-	offset = linear_page_index(dst_vma, dst_addr);
 	max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
-	if (unlikely(offset >= max_off))
+	if (unlikely(pgoff >= max_off))
 		goto out_release;
 
 	ret = shmem_add_to_page_cache(page, mapping, pgoff, NULL,
@@ -2439,7 +2440,7 @@ static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
 
 	ret = -EFAULT;
 	max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
-	if (unlikely(offset >= max_off))
+	if (unlikely(pgoff >= max_off))
 		goto out_release_unlock;
 
 	ret = -EEXIST;
@@ -2476,28 +2477,7 @@ static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
 	shmem_inode_unacct_blocks(inode, 1);
 	goto out;
 }
-
-int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm,
-			   pmd_t *dst_pmd,
-			   struct vm_area_struct *dst_vma,
-			   unsigned long dst_addr,
-			   unsigned long src_addr,
-			   struct page **pagep)
-{
-	return shmem_mfill_atomic_pte(dst_mm, dst_pmd, dst_vma,
-				      dst_addr, src_addr, false, pagep);
-}
-
-int shmem_mfill_zeropage_pte(struct mm_struct *dst_mm,
-			     pmd_t *dst_pmd,
-			     struct vm_area_struct *dst_vma,
-			     unsigned long dst_addr)
-{
-	struct page *page = NULL;
-
-	return shmem_mfill_atomic_pte(dst_mm, dst_pmd, dst_vma,
-				      dst_addr, 0, true, &page);
-}
+#endif /* CONFIG_USERFAULTFD */
 
 #ifdef CONFIG_TMPFS
 static const struct inode_operations shmem_symlink_inode_operations;
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index ce6cb4760d2c..60ae22207761 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -440,13 +440,9 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
 						 dst_vma, dst_addr);
 	} else {
 		VM_WARN_ON_ONCE(wp_copy);
-		if (!zeropage)
-			err = shmem_mcopy_atomic_pte(dst_mm, dst_pmd,
-						     dst_vma, dst_addr,
-						     src_addr, page);
-		else
-			err = shmem_mfill_zeropage_pte(dst_mm, dst_pmd,
-						       dst_vma, dst_addr);
+		err = shmem_mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma,
+					     dst_addr, src_addr, zeropage,
+					     page);
 	}
 
 	return err;
-- 
2.31.1.295.g9ea45b61b8-goog