+ writeback-only-update-writeback_index-for-range_cyclic-writeback.patch added to mm-unstable branch

+ writeback-only-update-writeback_index-for-range_cyclic-writeback.patch added to mm-unstable branch

[Andrew Morton]

The patch titled
     Subject: writeback: only update ->writeback_index for range_cyclic writeback
has been added to the -mm mm-unstable branch.  Its filename is
     writeback-only-update-writeback_index-for-range_cyclic-writeback.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/writeback-only-update-writeback_index-for-range_cyclic-writeback.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Christoph Hellwig <hch@lst.de>
Subject: writeback: only update ->writeback_index for range_cyclic writeback
Date: Thu, 15 Feb 2024 07:36:40 +0100

mapping->writeback_index is only [1] used as the starting point for
range_cyclic writeback, so there is no point in updating it for other
types of writeback.

[1] except for btrfs_defrag_file which does really odd things with
mapping->writeback_index.  But btrfs doesn't use write_cache_pages at all,
so this isn't relevant here.

Link: https://lkml.kernel.org/r/20240215063649.2164017-6-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Dave Chinner <dchinner@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Howells <dhowells@redhat.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/page-writeback.c |   24 ++++++++++++++----------
 1 file changed, 14 insertions(+), 10 deletions(-)

--- a/mm/page-writeback.c~writeback-only-update-writeback_index-for-range_cyclic-writeback
+++ a/mm/page-writeback.c
@@ -2403,7 +2403,6 @@ int write_cache_pages(struct address_spa
 	pgoff_t index;
 	pgoff_t end;		/* Inclusive */
 	pgoff_t done_index;
-	int range_whole = 0;
 	xa_mark_t tag;
 
 	folio_batch_init(&fbatch);
@@ -2413,8 +2412,6 @@ int write_cache_pages(struct address_spa
 	} else {
 		index = wbc->range_start >> PAGE_SHIFT;
 		end = wbc->range_end >> PAGE_SHIFT;
-		if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
-			range_whole = 1;
 	}
 	if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages) {
 		tag_pages_for_writeback(mapping, index, end);
@@ -2518,14 +2515,21 @@ continue_unlock:
 	}
 
 	/*
-	 * If we hit the last page and there is more work to be done: wrap
-	 * back the index back to the start of the file for the next
-	 * time we are called.
+	 * 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 && !done)
-		done_index = 0;
-	if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
-		mapping->writeback_index = done_index;
+	if (wbc->range_cyclic) {
+		if (done)
+			mapping->writeback_index = done_index;
+		else
+			mapping->writeback_index = 0;
+	}
 
 	return ret;
 }
_

Patches currently in -mm which might be from hch@lst.de are

writeback-dont-call-mapping_set_error-on-aop_writepage_activate.patch
writeback-fix-done_index-when-hitting-the-wbc-nr_to_write.patch
writeback-also-update-wbc-nr_to_write-on-writeback-failure.patch
writeback-only-update-writeback_index-for-range_cyclic-writeback.patch
writeback-rework-the-loop-termination-condition-in-write_cache_pages.patch
writeback-add-a-writeback-iterator.patch