writeback: account the number of pages written back

nr_to_write is a count of pages, so we need to decrease it by the number
of pages in the folio we just wrote, not by 1.  Most callers specify
either LONG_MAX or 1, so are unaffected, but writeback_sb_inodes() might
end up writing 512x as many pages as it asked for.

Dave added:

: XFS is the only filesystem this would affect, right?  AFAIA, nothing
: else enables large folios and uses writeback through
: write_cache_pages() at this point...
:
: In which case, I'd be surprised if much difference, if any, gets
: noticed by anyone.

Link: https://lkml.kernel.org/r/20230628185548.981888-1-willy@infradead.org
Fixes: 793917d997df ("mm/readahead: Add large folio readahead")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Jan Kara <jack@suse.cz>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 1d17fb1..d3f4200 100644 (file)
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2434,6 +2434,7 @@ int write_cache_pages(struct address_space *mapping,
 
                for (i = 0; i < nr_folios; i++) {
                        struct folio *folio = fbatch.folios[i];
+                       unsigned long nr;
 
                        done_index = folio->index;
 
@@ -2471,6 +2472,7 @@ continue_unlock:
 
                        trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));
                        error = writepage(folio, wbc, data);
+                       nr = folio_nr_pages(folio);
                        if (unlikely(error)) {
                                /*
                                 * Handle errors according to the type of
@@ -2489,8 +2491,7 @@ continue_unlock:
                                        error = 0;
                                } else if (wbc->sync_mode != WB_SYNC_ALL) {
                                        ret = error;
-                                       done_index = folio->index +
-                                               folio_nr_pages(folio);
+                                       done_index = folio->index + nr;
                                        done = 1;
                                        break;
                                }
@@ -2504,7 +2505,8 @@ continue_unlock:
                         * keep going until we have written all the pages
                         * we tagged for writeback prior to entering this loop.
                         */
-                       if (--wbc->nr_to_write <= 0 &&
+                       wbc->nr_to_write -= nr;
+                       if (wbc->nr_to_write <= 0 &&
                            wbc->sync_mode == WB_SYNC_NONE) {
                                done = 1;
                                break;