Re: [PATCH 0/5] fs/buffer: strack reduction on async read

[Matthew Wilcox <willy@infradead.org>]

On Tue, Dec 17, 2024 at 06:26:21PM -0800, Luis Chamberlain wrote:
> This splits up a minor enhancement from the bs > ps device support
> series into its own series for better review / focus / testing.
> This series just addresses the reducing the array size used and cleaning
> up the async read to be easier to read and maintain.

How about this approach instead -- get rid of the batch entirely?

diff --git a/fs/buffer.c b/fs/buffer.c
index cc8452f60251..f50ebbc1f518 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2361,9 +2361,9 @@ int block_read_full_folio(struct folio *folio, get_block_t *get_block)
 {
 	struct inode *inode = folio->mapping->host;
 	sector_t iblock, lblock;
-	struct buffer_head *bh, *head, *arr[MAX_BUF_PER_PAGE];
+	struct buffer_head *bh, *head;
 	size_t blocksize;
-	int nr, i;
+	int i, submitted = 0;
 	int fully_mapped = 1;
 	bool page_error = false;
 	loff_t limit = i_size_read(inode);
@@ -2380,7 +2380,6 @@ int block_read_full_folio(struct folio *folio, get_block_t *get_block)
 	iblock = div_u64(folio_pos(folio), blocksize);
 	lblock = div_u64(limit + blocksize - 1, blocksize);
 	bh = head;
-	nr = 0;
 	i = 0;
 
 	do {
@@ -2411,40 +2410,30 @@ int block_read_full_folio(struct folio *folio, get_block_t *get_block)
 			if (buffer_uptodate(bh))
 				continue;
 		}
-		arr[nr++] = bh;
+
+		lock_buffer(bh);
+		if (buffer_uptodate(bh)) {
+			unlock_buffer(bh);
+			continue;
+		}
+
+		mark_buffer_async_read(bh);
+		submit_bh(REQ_OP_READ, bh);
+		submitted++;
 	} while (i++, iblock++, (bh = bh->b_this_page) != head);
 
 	if (fully_mapped)
 		folio_set_mappedtodisk(folio);
 
-	if (!nr) {
-		/*
-		 * All buffers are uptodate or get_block() returned an
-		 * error when trying to map them - we can finish the read.
-		 */
-		folio_end_read(folio, !page_error);
-		return 0;
-	}
-
-	/* Stage two: lock the buffers */
-	for (i = 0; i < nr; i++) {
-		bh = arr[i];
-		lock_buffer(bh);
-		mark_buffer_async_read(bh);
-	}
-
 	/*
-	 * Stage 3: start the IO.  Check for uptodateness
-	 * inside the buffer lock in case another process reading
-	 * the underlying blockdev brought it uptodate (the sct fix).
+	 * All buffers are uptodate or get_block() returned an error
+	 * when trying to map them - we must finish the read because
+	 * end_buffer_async_read() will never be called on any buffer
+	 * in this folio.
 	 */
-	for (i = 0; i < nr; i++) {
-		bh = arr[i];
-		if (buffer_uptodate(bh))
-			end_buffer_async_read(bh, 1);
-		else
-			submit_bh(REQ_OP_READ, bh);
-	}
+	if (!submitted)
+		folio_end_read(folio, !page_error);
+
 	return 0;
 }
 EXPORT_SYMBOL(block_read_full_folio);