shmem: convert shmem_mfill_atomic_pte() to use a folio

Assert that this is a single-page folio as there are several assumptions
in here that it's exactly PAGE_SIZE bytes large.  Saves several calls to
compound_head() and removes the last caller of shmem_alloc_page().

Link: https://lkml.kernel.org/r/20220902194653.1739778-18-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/shmem.c

@@ -2374,12 +2374,6 @@ static struct inode *shmem_get_inode(struct super_block *sb, struct inode *dir,
 }
 
 #ifdef CONFIG_USERFAULTFD
-static struct page *shmem_alloc_page(gfp_t gfp,
-			struct shmem_inode_info *info, pgoff_t index)
-{
-	return &shmem_alloc_folio(gfp, info, index)->page;
-}
-
 int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
 			   pmd_t *dst_pmd,
 			   struct vm_area_struct *dst_vma,
@@ -2395,7 +2389,6 @@ int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
 	pgoff_t pgoff = linear_page_index(dst_vma, dst_addr);
 	void *page_kaddr;
 	struct folio *folio;
-	struct page *page;
 	int ret;
 	pgoff_t max_off;
 
@@ -2414,53 +2407,53 @@ int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
 
 	if (!*pagep) {
 		ret = -ENOMEM;
-		page = shmem_alloc_page(gfp, info, pgoff);
-		if (!page)
+		folio = shmem_alloc_folio(gfp, info, pgoff);
+		if (!folio)
 			goto out_unacct_blocks;
 
 		if (!zeropage) {	/* COPY */
-			page_kaddr = kmap_atomic(page);
+			page_kaddr = kmap_local_folio(folio, 0);
 			ret = copy_from_user(page_kaddr,
 					     (const void __user *)src_addr,
 					     PAGE_SIZE);
-			kunmap_atomic(page_kaddr);
+			kunmap_local(page_kaddr);
 
 			/* fallback to copy_from_user outside mmap_lock */
 			if (unlikely(ret)) {
-				*pagep = page;
+				*pagep = &folio->page;
 				ret = -ENOENT;
 				/* don't free the page */
 				goto out_unacct_blocks;
 			}
 
-			flush_dcache_page(page);
+			flush_dcache_folio(folio);
 		} else {		/* ZEROPAGE */
-			clear_user_highpage(page, dst_addr);
+			clear_user_highpage(&folio->page, dst_addr);
 		}
 	} else {
-		page = *pagep;
+		folio = page_folio(*pagep);
+		VM_BUG_ON_FOLIO(folio_test_large(folio), folio);
 		*pagep = NULL;
 	}
 
-	VM_BUG_ON(PageLocked(page));
-	VM_BUG_ON(PageSwapBacked(page));
-	__SetPageLocked(page);
-	__SetPageSwapBacked(page);
-	__SetPageUptodate(page);
+	VM_BUG_ON(folio_test_locked(folio));
+	VM_BUG_ON(folio_test_swapbacked(folio));
+	__folio_set_locked(folio);
+	__folio_set_swapbacked(folio);
+	__folio_mark_uptodate(folio);
 
 	ret = -EFAULT;
 	max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
 	if (unlikely(pgoff >= max_off))
 		goto out_release;
 
-	folio = page_folio(page);
 	ret = shmem_add_to_page_cache(folio, mapping, pgoff, NULL,
 				      gfp & GFP_RECLAIM_MASK, dst_mm);
 	if (ret)
 		goto out_release;
 
 	ret = mfill_atomic_install_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
-				       page, true, wp_copy);
+				       &folio->page, true, wp_copy);
 	if (ret)
 		goto out_delete_from_cache;
 
@@ -2470,13 +2463,13 @@ int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
 	shmem_recalc_inode(inode);
 	spin_unlock_irq(&info->lock);
 
-	unlock_page(page);
+	folio_unlock(folio);
 	return 0;
 out_delete_from_cache:
-	delete_from_page_cache(page);
+	filemap_remove_folio(folio);
 out_release:
-	unlock_page(page);
-	put_page(page);
+	folio_unlock(folio);
+	folio_put(folio);
 out_unacct_blocks:
 	shmem_inode_unacct_blocks(inode, 1);
 	return ret;