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Date:   Sat, 15 Apr 2023 04:44:22 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Luis Chamberlain <mcgrof@kernel.org>
Cc:     Hannes Reinecke <hare@suse.de>,
        Pankaj Raghav <p.raghav@samsung.com>, brauner@kernel.org,
        viro@zeniv.linux.org.uk, akpm@linux-foundation.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        gost.dev@samsung.com
Subject: Re: [RFC 0/4] convert create_page_buffers to create_folio_buffers
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On Fri, Apr 14, 2023 at 08:24:56PM -0700, Luis Chamberlain wrote:
> I thought of that but I saw that the loop that assigns the arr only
> pegs a bh if we don't "continue" for certain conditions, which made me
> believe that we only wanted to keep on the array as non-null items which
> meet the initial loop's criteria. If that is not accurate then yes,
> the simplication is nice!

Uh, right.  A little bit more carefully this time ... how does this
look?

diff --git a/fs/buffer.c b/fs/buffer.c
index 5e67e21b350a..dff671079b02 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2282,7 +2282,7 @@ 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;
 	unsigned int blocksize, bbits;
 	int nr, i;
 	int fully_mapped = 1;
@@ -2335,7 +2335,7 @@ int block_read_full_folio(struct folio *folio, get_block_t *get_block)
 			if (buffer_uptodate(bh))
 				continue;
 		}
-		arr[nr++] = bh;
+		nr++;
 	} while (i++, iblock++, (bh = bh->b_this_page) != head);
 
 	if (fully_mapped)
@@ -2352,25 +2352,29 @@ int block_read_full_folio(struct folio *folio, get_block_t *get_block)
 		return 0;
 	}
 
-	/* Stage two: lock the buffers */
-	for (i = 0; i < nr; i++) {
-		bh = arr[i];
+	/*
+	 * Stage two: lock the buffers.  Recheck the uptodate flag under
+	 * the lock in case somebody else brought it uptodate first.
+	 */
+	bh = head;
+	do {
+		if (buffer_uptodate(bh))
+			continue;
 		lock_buffer(bh);
+		if (buffer_uptodate(bh)) {
+			unlock_buffer(bh);
+			continue;
+		}
 		mark_buffer_async_read(bh);
-	}
+	} while ((bh = bh->b_this_page) != head);
 
-	/*
-	 * 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).
-	 */
-	for (i = 0; i < nr; i++) {
-		bh = arr[i];
-		if (buffer_uptodate(bh))
-			end_buffer_async_read(bh, 1);
-		else
+	/* Stage 3: start the IO */
+	bh = head;
+	do {
+		if (buffer_async_read(bh))
 			submit_bh(REQ_OP_READ, bh);
-	}
+	} while ((bh = bh->b_this_page) != head);
+
 	return 0;
 }
 EXPORT_SYMBOL(block_read_full_folio);


I do wonder how much it's worth doing this vs switching to non-BH methods.
I appreciate that's a lot of work still.