[PATCH 09/19] writeback: Simplify the loops in write_cache_pages()

Christoph Hellwig
2024-01-25T08:58:47Z

From: "Matthew Wilcox (Oracle)" <willy@infradead.org>

Collapse the two nested loops into one. This is needed as a step
towards turning this into an iterator.

Note that this drops the "index <= end" check in the previous outer loop
and just relies on filemap_get_folios_tag() to return 0 entries when
index > end. This actually has a subtle implication when end == -1
because then the returned index will be -1 as well and thus if there is
page present on index -1, we could be looping indefinitely. But as the
comment in filemap_get_folios_tag documents this as already broken anyway
we should not worry about it here either. The fix for that would
probably a change to the filemap_get_folios_tag() calling convention.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
[hch: updated the commit log based on feedback from Jan Kara]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Dave Chinner <dchinner@redhat.com>
---
mm/page-writeback.c | 94 ++++++++++++++++++++++-----------------------
1 file changed, 46 insertions(+), 48 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index cec683c7217d2e..d6ac414ddce9ca 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2475,6 +2475,7 @@ int write_cache_pages(struct address_space *mapping,
{
int error;
pgoff_t end; /* Inclusive */
+ int i = 0;

if (wbc->range_cyclic) {
wbc->index = mapping->writeback_index; /* prev offset */
@@ -2489,63 +2490,60 @@ int write_cache_pages(struct address_space *mapping,
folio_batch_init(&wbc->fbatch);
wbc->err = 0;

- while (wbc->index <= end) {
- int i;
-
- writeback_get_batch(mapping, wbc);
+ for (;;) {
+ struct folio *folio;
+ unsigned long nr;

+ if (i == wbc->fbatch.nr) {
+ writeback_get_batch(mapping, wbc);
+ i = 0;
+ }
if (wbc->fbatch.nr == 0)
break;

- for (i = 0; i < wbc->fbatch.nr; i++) {
- struct folio *folio = wbc->fbatch.folios[i];
- unsigned long nr;
+ folio = wbc->fbatch.folios[i++];

- folio_lock(folio);
- if (!folio_prepare_writeback(mapping, wbc, folio)) {
- folio_unlock(folio);
- continue;
- }
+ folio_lock(folio);
+ if (!folio_prepare_writeback(mapping, wbc, folio)) {
+ folio_unlock(folio);
+ continue;
+ }

- trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));
+ trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));

- error = writepage(folio, wbc, data);
- nr = folio_nr_pages(folio);
- wbc->nr_to_write -= nr;
+ error = writepage(folio, wbc, data);
+ nr = folio_nr_pages(folio);
+ wbc->nr_to_write -= nr;

- /*
- * Handle the legacy AOP_WRITEPAGE_ACTIVATE magic return
- * value. Eventually all instances should just unlock
- * the folio themselves and return 0;
- */
- if (error == AOP_WRITEPAGE_ACTIVATE) {
- folio_unlock(folio);
- error = 0;
- }
-
- if (error && !wbc->err)
- wbc->err = error;
+ /*
+ * Handle the legacy AOP_WRITEPAGE_ACTIVATE magic return value.
+ * Eventually all instances should just unlock the folio
+ * themselves and return 0;
+ */
+ if (error == AOP_WRITEPAGE_ACTIVATE) {
+ folio_unlock(folio);
+ error = 0;
+ }

- /*
- * For integrity sync we have to keep going until we
- * have written all the folios we tagged for writeback
- * prior to entering this loop, even if we run past
- * wbc->nr_to_write or encounter errors. This is
- * because the file system may still have state to clear
- * for each folio. We'll eventually return the first
- * error encountered.
- *
- * 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.
- */
- if (wbc->sync_mode == WB_SYNC_NONE &&
- (wbc->err || wbc->nr_to_write <= 0)) {
- writeback_finish(mapping, wbc,
- folio->index + nr);
- return error;
- }
+ if (error && !wbc->err)
+ wbc->err = error;
+
+ /*
+ * For integrity sync we have to keep going until we have
+ * written all the folios we tagged for writeback prior to
+ * entering this loop, even if we run past wbc->nr_to_write or
+ * encounter errors. This is because the file system may still
+ * have state to clear for each folio. We'll eventually return
+ * the first error encountered.
+ *
+ * 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.
+ */
+ if (wbc->sync_mode == WB_SYNC_NONE &&
+ (wbc->err || wbc->nr_to_write <= 0)) {
+ writeback_finish(mapping, wbc, folio->index + nr);
+ return error;
}
}

--
2.39.2