writeback: pay attention to wbc->nr_to_write in write_cache_pages

If a filesystem writes more than one page in ->writepage, write_cache_pages
fails to notice this and continues to attempt writeback when wbc->nr_to_write
has gone negative - this trace was captured from XFS:

    wbc_writeback_start: towrt=1024
    wbc_writepage: towrt=1024
    wbc_writepage: towrt=0
    wbc_writepage: towrt=-1
    wbc_writepage: towrt=-5
    wbc_writepage: towrt=-21
    wbc_writepage: towrt=-85

This has adverse effects on filesystem writeback behaviour. write_cache_pages()
needs to terminate after a certain number of pages are written, not after a
certain number of calls to ->writepage are made.  This is a regression
introduced by 17bc6c30cf6bfffd816bdc53682dd46fc34a2cf4 ("vfs: Add
no_nrwrite_index_update writeback control flag"), but cannot be reverted
directly due to subsequent bug fixes that have gone in on top of it.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 10 deletions

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 5fa63bdf52e4..b3dbb8040ed5 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -835,7 +835,6 @@ int write_cache_pages(struct address_space *mapping,
         pgoff_t done_index;
         int cycled;
         int range_whole = 0;
-        long nr_to_write = wbc->nr_to_write;
 
         pagevec_init(&pvec, 0);
         if (wbc->range_cyclic) {
@@ -935,11 +934,10 @@ continue_unlock:
                                         done = 1;
                                         break;
                                 }
-                        }
+                        }
 
-                        if (nr_to_write > 0) {
-                                nr_to_write--;
-                                if (nr_to_write == 0 &&
+                        if (wbc->nr_to_write > 0) {
+                                if (--wbc->nr_to_write == 0 &&
                                     wbc->sync_mode == WB_SYNC_NONE) {
                                         /*
                                          * We stop writing back only if we are
@@ -970,11 +968,8 @@ continue_unlock:
                 end = writeback_index - 1;
                 goto retry;
         }
-        if (!wbc->no_nrwrite_index_update) {
-                if (wbc->range_cyclic || (range_whole && nr_to_write > 0))
-                        mapping->writeback_index = done_index;
-                wbc->nr_to_write = nr_to_write;
-        }
+        if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
+                mapping->writeback_index = done_index;
 
         return ret;
 }