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/[email protected]
Fixes: 793917d ("mm/readahead: Add large folio readahead")
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Dave Chinner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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 @@ int write_cache_pages(struct address_space *mapping,
 
 			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 @@ int write_cache_pages(struct address_space *mapping,
 					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 @@ int write_cache_pages(struct address_space *mapping,
 			 * 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;