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>
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 5fa63bd..b3dbb80 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -835,7 +835,6 @@
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 @@
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 @@
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;
}