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

Hugh Dickins
2010-12-21T06:00:01Z

On Fri, 17 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.
>
> CC: Hugh Dickins <hughd@google.com>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

Whilst I do like this change, and I do think it's the right thing to do
(given that the bdi has explicitly opted out of what it then got into),
I've a sneaking feeling that something somewhere may show a regression
from it. IIRC, there were circumstances in which it actually did
(inadvertently) end up throttling the tmpfs writing - if there were
too many dirty non-tmpfs pages around??

What am I saying?! I think I'm asking you to look more closely at what
actually used to happen, and be more explicit about the behavior you're
stopping here - although the patch is mainly code optimization, there
is some functional change I think. (You do mention throttling on
tmpfs/ramfs, but the way it worked out wasn't straightforward.)

I'd better not burble on for a third paragraph!

Hugh

> ---
> mm/page-writeback.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> --- linux-next.orig/mm/page-writeback.c 2010-12-17 19:09:19.000000000 +0800
> +++ linux-next/mm/page-writeback.c 2010-12-17 19:09:22.000000000 +0800
> @@ -899,6 +899,9 @@ 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))