Re: [PATCH 4/6] writeback: pay attention to wbc->nr_to_write in write_cache_pages

Andrew Morton
2010-05-27T21:31:18Z

On Tue, 25 May 2010 20:54:10 +1000
Dave Chinner <david@fromorbit.com> wrote:

> From: Dave Chinner <dchinner@redhat.com>
>
> 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, but cannot be reverted

It's conventional to identify commits by their title as well as their
hash. So 17bc6c30cf6bfffd816bdc53682dd46fc34a2cf4 ("vfs: Add
no_nrwrite_index_update writeback control flag"). Because that commit
might have different hashes in different trees, I think. A Linus idea.

I do this ten times a day - It's a PITA.

> directly due to subsequent bug fixes that have gone in on top of it.
>
> This commit adds a ->writepage tracepoint inside write_cache_pages() (how the
> above trace was generated) and does the revert manually leaving the subsequent
> bug fixes in tact. ext4 is not affected by this as a previous commit in the

"intact".

> series stops ext4 from using the generic function.
>
> - 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
> @@ -974,11 +973,8 @@ 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;

'bout time we fixed that. I wonder why it took so long to find.