buffer: avoid tail commit walk for uptodate folios

block_commit_write() always walks every buffer_head attached to the
folio.  That was cheap for order-0 folios, but large folios can contain
hundreds of buffer_heads.  For a small buffered overwrite of an
already-uptodate large folio, the commit work is therefore proportional
to the folio size rather than the copied range.

This became visible with ext4 regular-file large folios, where cached
small overwrites reach block_commit_write() through block_write_end().
Before ext4 enabled large folios for regular files, this path was only
hit with order-0 folios for normal ext4 buffered writes, so the full walk
was bounded.  The ext4 large-folio commit is therefore the regression
point for this generic helper cost.

The full walk is still needed when the folio is not uptodate, because
block_commit_write() uses per-buffer uptodate state to decide whether
the whole folio can be marked uptodate.  Keep those folios on the old
full-buffer path.

For a folio that was already uptodate on entry, the commit no longer
needs tail buffers for folio-uptodate discovery.  The copied range has
already been processed once block_start reaches @to, so stop there and
avoid the suffix walk.

Fixes: 7ac67301e8 ("ext4: enable large folio for regular file")
Suggested-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: stable@vger.kernel.org # v6.16+
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jia Zhu <zhujia.zj@bytedance.com>
Link: https://patch.msgid.link/20260609035202.90669-2-zhujia.zj@bytedance.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/buffer.c

@@ -2177,6 +2177,7 @@ void block_commit_write(struct folio *folio, size_t from, size_t to)
 {
 	size_t block_start, block_end;
 	bool partial = false;
+	bool uptodate = folio_test_uptodate(folio);
 	unsigned blocksize;
 	struct buffer_head *bh, *head;
 
@@ -2199,6 +2200,8 @@ void block_commit_write(struct folio *folio, size_t from, size_t to)
 			clear_buffer_new(bh);
 
 		block_start = block_end;
+		if (uptodate && block_start >= to)
+			break;
 		bh = bh->b_this_page;
 	} while (bh != head);