Re: [PATCH] writeback: skip balance_dirty_pages() for in-memory fs

Hugh Dickins
2010-12-30T03:15:55Z

On Tue, 21 Dec 2010, Wu Fengguang wrote:
>
> This avoids unnecessary checks and dirty throttling on tmpfs/ramfs.
>
> It also prevents
>
> [ 388.126563] BUG: unable to handle kernel NULL pointer dereference at 0000000000000050
>
> in the balance_dirty_pages tracepoint, which will call
>
> dev_name(mapping->backing_dev_info->dev)
>
> but shmem_backing_dev_info.dev is NULL.
>
> Summary notes about the tmpfs/ramfs behavior changes:
>
> As for 2.6.36 and older kernels, the tmpfs writes will sleep inside
> balance_dirty_pages() as long as we are over the (dirty+background)/2
> global throttle threshold. This is because both the dirty pages and
> threshold will be 0 for tmpfs/ramfs. Hence this test will always
> evaluate to TRUE:
>
> dirty_exceeded =
> (bdi_nr_reclaimable + bdi_nr_writeback >= bdi_thresh)
> || (nr_reclaimable + nr_writeback >= dirty_thresh);
>
> For 2.6.37, someone complained that the current logic does not allow the
> users to set vm.dirty_ratio=0. So commit 4cbec4c8b9 changed the test to
>
> dirty_exceeded =
> (bdi_nr_reclaimable + bdi_nr_writeback > bdi_thresh)
> || (nr_reclaimable + nr_writeback > dirty_thresh);
>
> So 2.6.37 will behave differently for tmpfs/ramfs: it will never get
> throttled unless the global dirty threshold is exceeded (which is very
> unlikely to happen; once happen, will block many tasks).
>
> I'd say that the 2.6.36 behavior is very bad for tmpfs/ramfs. It means
> for a busy writing server, tmpfs write()s may get livelocked! The
> "inadvertent" throttling can hardly bring help to any workload because
> of its "either no throttling, or get throttled to death" property.
>
> So based on 2.6.37, this patch won't bring more noticeable changes.
>
> CC: Hugh Dickins <hughd@google.com>
> CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Acked-by: Rik van Riel <riel@redhat.com>
> Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

Thanks a lot for investigating further and writing it all up here.

Acked-by: Hugh Dickins <hughd@google.com>

I notice bdi_cap_writeback_dirty go from bdi_writeout_fraction(), and
bdi_cap_account_dirty appear in balance_dirty_pages_ratelimited_nr():
maybe one day a patch to use just one flag throughout? Unless you can
dream up a use for the divergence. (I hate wasting brainpower trying to
decide which of two always-the-sames to use, like page_cache_release()
and put_page(), until there's actual code to distinguish them.)

Hugh

> ---
> mm/page-writeback.c | 10 ++++------
> 1 file changed, 4 insertions(+), 6 deletions(-)
>
> --- linux-next.orig/mm/page-writeback.c 2010-12-18 09:14:53.000000000 +0800
> +++ linux-next/mm/page-writeback.c 2010-12-21 17:35:44.000000000 +0800
> @@ -230,13 +230,8 @@ void task_dirty_inc(struct task_struct *
> static void bdi_writeout_fraction(struct backing_dev_info *bdi,
> long *numerator, long *denominator)
> {
> - if (bdi_cap_writeback_dirty(bdi)) {
> - prop_fraction_percpu(&vm_completions, &bdi->completions,
> + prop_fraction_percpu(&vm_completions, &bdi->completions,
> numerator, denominator);
> - } else {
> - *numerator = 0;
> - *denominator = 1;
> - }
> }
>
> static inline void task_dirties_fraction(struct task_struct *tsk,
> @@ -878,6 +873,9 @@ void balance_dirty_pages_ratelimited_nr(
> {
> struct backing_dev_info *bdi = mapping->backing_dev_info;
>
> + if (!bdi_cap_account_dirty(bdi))
> + return;
> +
> current->nr_dirtied += nr_pages_dirtied;
>
> if (unlikely(!current->nr_dirtied_pause))