Squashfs: Rewrite and update squashfs_readahead_fragment() to not use page->index

The previous implementation lacked error checking (e.g. the bytes
returned by squashfs_fill_page() is not checked), and the use of
page->index could not be removed without substantially rewriting
the routine to use the page actor abstraction used elsewhere.

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
Link: https://lore.kernel.org/r/20240818235847.170468-5-phillip@squashfs.org.uk
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: plougher

fs/squashfs/file.c

@@ -494,39 +494,73 @@ static int squashfs_read_folio(struct file *file, struct folio *folio)
 }
 
 static int squashfs_readahead_fragment(struct page **page,
-	unsigned int pages, unsigned int expected)
+	unsigned int pages, unsigned int expected, loff_t start)
 {
 	struct inode *inode = page[0]->mapping->host;
 	struct squashfs_cache_entry *buffer = squashfs_get_fragment(inode->i_sb,
 		squashfs_i(inode)->fragment_block,
 		squashfs_i(inode)->fragment_size);
 	struct squashfs_sb_info *msblk = inode->i_sb->s_fs_info;
-	unsigned int n, mask = (1 << (msblk->block_log - PAGE_SHIFT)) - 1;
-	int error = buffer->error;
+	int i, bytes, copied;
+	struct squashfs_page_actor *actor;
+	unsigned int offset;
+	void *addr;
+	struct page *last_page;
 
-	if (error)
+	if (buffer->error)
 		goto out;
 
-	expected += squashfs_i(inode)->fragment_offset;
+	actor = squashfs_page_actor_init_special(msblk, page, pages,
+							expected, start);
+	if (!actor)
+		goto out;
 
-	for (n = 0; n < pages; n++) {
-		unsigned int base = (page[n]->index & mask) << PAGE_SHIFT;
-		unsigned int offset = base + squashfs_i(inode)->fragment_offset;
+	squashfs_actor_nobuff(actor);
+	addr = squashfs_first_page(actor);
 
-		if (expected > offset) {
-			unsigned int avail = min_t(unsigned int, expected -
-				offset, PAGE_SIZE);
+	for (copied = offset = 0; offset < expected; offset += PAGE_SIZE) {
+		int avail = min_t(int, expected - offset, PAGE_SIZE);
 
-			squashfs_fill_page(page[n], buffer, offset, avail);
+		if (!IS_ERR(addr)) {
+			bytes = squashfs_copy_data(addr, buffer, offset +
+					squashfs_i(inode)->fragment_offset, avail);
+
+			if (bytes != avail)
+				goto failed;
 		}
 
-		unlock_page(page[n]);
-		put_page(page[n]);
+		copied += avail;
+		addr = squashfs_next_page(actor);
 	}
 
+	last_page = squashfs_page_actor_free(actor);
+
+	if (copied == expected) {
+		/* Last page (if present) may have trailing bytes not filled */
+		bytes = copied % PAGE_SIZE;
+		if (bytes && last_page)
+			memzero_page(last_page, bytes, PAGE_SIZE - bytes);
+
+		for (i = 0; i < pages; i++) {
+			flush_dcache_page(page[i]);
+			SetPageUptodate(page[i]);
+		}
+	}
+
+	for (i = 0; i < pages; i++) {
+		unlock_page(page[i]);
+		put_page(page[i]);
+	}
+
+	squashfs_cache_put(buffer);
+	return 0;
+
+failed:
+	squashfs_page_actor_free(actor);
+
 out:
 	squashfs_cache_put(buffer);
-	return error;
+	return 1;
 }
 
 static void squashfs_readahead(struct readahead_control *ractl)
@@ -572,7 +606,7 @@ static void squashfs_readahead(struct readahead_control *ractl)
 		if (start >> msblk->block_log == file_end &&
 				squashfs_i(inode)->fragment_block != SQUASHFS_INVALID_BLK) {
 			res = squashfs_readahead_fragment(pages, nr_pages,
-							  expected);
+							  expected, start);
 			if (res)
 				goto skip_pages;
 			continue;