linux-kernel - Re: [PATCH 19/19] writeback: simplify writeback iteration

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Message-ID: <20240130142205.GB31330@lst.de>
Date: Tue, 30 Jan 2024 15:22:05 +0100
From: Christoph Hellwig <hch@....de>
To: Jan Kara <jack@...e.cz>
Cc: Christoph Hellwig <hch@....de>, linux-mm@...ck.org,
	Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.com>,
	David Howells <dhowells@...hat.com>,
	Brian Foster <bfoster@...hat.com>,
	Christian Brauner <brauner@...nel.org>,
	"Darrick J. Wong" <djwong@...nel.org>, linux-xfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 19/19] writeback: simplify writeback iteration

On Tue, Jan 30, 2024 at 03:16:01PM +0100, Christoph Hellwig wrote:
> On Tue, Jan 30, 2024 at 11:46:05AM +0100, Jan Kara wrote:
> > Looking at it now I'm thinking whether we would not be better off to
> > completely dump the 'error' argument of writeback_iter() /
> > writeback_iter_next() and just make all .writepage implementations set
> > wbc->err directly. But that means touching all the ~20 writepage
> > implementations we still have...
> 
> Heh.  I actually had an earlier version that looked at wbc->err in
> the ->writepages callers.  But it felt a bit too ugly.
> 
> > > +		 */
> > > +		if (wbc->sync_mode == WB_SYNC_NONE &&
> > > +		    (wbc->err || wbc->nr_to_write <= 0))
> > > +			goto finish;
> > 
> > I think it would be a bit more comprehensible if we replace the goto with:
> > 			folio_batch_release(&wbc->fbatch);
> > 			if (wbc->range_cyclic)
> > 				mapping->writeback_index =
> > 					folio->index + folio_nr_pages(folio);
> > 			*error = wbc->err;
> > 			return NULL;
> 
> I agree that keeping the logic on when to break and when to set the
> writeback_index is good, but duplicating the batch release and error
> assignment seems a bit suboptimal.  Let me know what you think of the
> alternatіve variant below.

And now for real:


diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index d8fcbac2d72310..3e4aa5bfe75819 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2475,10 +2475,23 @@ struct folio *writeback_iter(struct address_space *mapping,
 		 * For background writeback just push done_index past this folio
 		 * so that we can just restart where we left off and media
 		 * errors won't choke writeout for the entire file.
+		 *
+		 * For range cyclic writeback we need to remember where we
+		 * stopped so that we can continue there next time we are
+		 * called.  If we hit the last page and there is more work
+		 * to be done, wrap back to the start of the file.
+		 *
+		 * For non-cyclic writeback we always start looking up at the
+		 * beginning of the file if we are called again, which can only
+		 * happen due to -ENOMEM from the file system.
 		 */
 		if (wbc->sync_mode == WB_SYNC_NONE &&
-		    (wbc->err || wbc->nr_to_write <= 0))
+		    (wbc->err || wbc->nr_to_write <= 0)) {
+			if (wbc->range_cyclic)
+				mapping->writeback_index =
+					folio->index + folio_nr_pages(folio);
 			goto finish;
+		}
 	} else {
 		if (wbc->range_cyclic)
 			wbc->index = mapping->writeback_index; /* prev offset */
@@ -2492,31 +2505,15 @@ struct folio *writeback_iter(struct address_space *mapping,
 	}
 
 	folio = writeback_get_folio(mapping, wbc);
-	if (!folio)
+	if (!folio) {
+		if (wbc->range_cyclic)
+			mapping->writeback_index = 0;
 		goto finish;
+	}
 	return folio;
 
 finish:
 	folio_batch_release(&wbc->fbatch);
-
-	/*
-	 * For range cyclic writeback we need to remember where we stopped so
-	 * that we can continue there next time we are called.  If  we hit the
-	 * last page and there is more work to be done, wrap back to the start
-	 * of the file.
-	 *
-	 * For non-cyclic writeback we always start looking up at the beginning
-	 * of the file if we are called again, which can only happen due to
-	 * -ENOMEM from the file system.
-	 */
-	if (wbc->range_cyclic) {
-		WARN_ON_ONCE(wbc->sync_mode != WB_SYNC_NONE);
-		if (wbc->err || wbc->nr_to_write <= 0)
-			mapping->writeback_index =
-				folio->index + folio_nr_pages(folio);
-		else
-			mapping->writeback_index = 0;
-	}
 	*error = wbc->err;
 	return NULL;
 }