From: Tejun Heo tj@kernel.org
commit f7bddf1e27d18fbc7d3e3056ba449cfbe4e20b0a upstream.
During a cyclic writeback, extent_write_cache_pages() uses done_index to update the writeback_index after the current run is over. However, instead of current index + 1, it gets to to the current index itself.
Unfortunately, this, combined with returning on EOF instead of looping back, can lead to the following pathlogical behavior.
1. There is a single file which has accumulated enough dirty pages to trigger balance_dirty_pages() and the writer appending to the file with a series of short writes.
2. balance_dirty_pages kicks in, wakes up background writeback and sleeps.
3. Writeback kicks in and the cursor is on the last page of the dirty file. Writeback is started or skipped if already in progress. As it's EOF, extent_write_cache_pages() returns and the cursor is set to done_index which is pointing to the last page.
4. Writeback is done. Nothing happens till balance_dirty_pages finishes, at which point we go back to #1.
This can almost completely stall out writing back of the file and keep the system over dirty threshold for a long time which can mess up the whole system. We encountered this issue in production with a package handling application which can reliably reproduce the issue when running under tight memory limits.
Reading the comment in the error handling section, this seems to be to avoid accidentally skipping a page in case the write attempt on the page doesn't succeed. However, this concern seems bogus.
On each page, the code either:
* Skips and moves onto the next page.
* Fails issue and sets done_index to index + 1.
* Successfully issues and continue to the next page if budget allows and not EOF.
IOW, as long as it's not EOF and there's budget, the code never retries writing back the same page. Only when a page happens to be the last page of a particular run, we end up retrying the page, which can't possibly guarantee anything data integrity related. Besides, cyclic writes are only used for non-syncing writebacks meaning that there's no data integrity implication to begin with.
Fix it by always setting done_index past the current page being processed.
Note that this problem exists in other writepages too.
CC: stable@vger.kernel.org # 4.19+ Signed-off-by: Tejun Heo tj@kernel.org Reviewed-by: David Sterba dsterba@suse.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/btrfs/extent_io.c | 12 +----------- 1 file changed, 1 insertion(+), 11 deletions(-)
--- a/fs/btrfs/extent_io.c +++ b/fs/btrfs/extent_io.c @@ -3956,7 +3956,7 @@ retry: for (i = 0; i < nr_pages; i++) { struct page *page = pvec.pages[i];
- done_index = page->index; + done_index = page->index + 1; /* * At this point we hold neither the i_pages lock nor * the page lock: the page may be truncated or @@ -3993,16 +3993,6 @@ retry: ret = 0; } if (ret < 0) { - /* - * done_index is set past this page, - * so media errors will not choke - * background writeout for the entire - * file. This has consequences for - * range_cyclic semantics (ie. it may - * not be suitable for data integrity - * writeout). - */ - done_index = page->index + 1; done = 1; break; }