Re: [PATCH 2/6] MM: annotate congestion_wait() and wait_iff_congested() as ineffective.

[NeilBrown]

On Wed, 15 Sep 2021, Michal Hocko wrote:
> On Tue 14-09-21 10:13:04, Neil Brown wrote:
> > Only 4 subsystems call set_bdi_congested() or clear_bdi_congested():
> >  block/pktcdvd, fs/ceph fs/fuse fs/nfs
> > 
> > It may make sense to use congestion_wait() or wait_iff_congested()
> > within these subsystems, but they have no value outside of these.
> > 
> > Add documentation comments to these functions to discourage further use.
> 
> This is an unfortunate state. The MM layer still relies on the API.
> While adding a documentation to clarify the current status can stop more
> usage I am wondering what is a real alternative. My experience tells me
> that a lack of real alternative will lead to new creative ways of doing
> things instead.

That is a valid concern.  Discouraging the use of an interface without
providing a clear alternative risks people doing worse things.

At lease if people continue to use congestion_wait(), then we will be
able to find those uses when we are able to provide a better approach.

I'll drop this patch.

Thanks,
NeilBrown


>  
> > Signed-off-by: NeilBrown <neilb@xxxxxxx>
> > ---
> >  include/linux/backing-dev.h |    7 +++++++
> >  mm/backing-dev.c            |    9 +++++++++
> >  2 files changed, 16 insertions(+)
> > 
> > diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
> > index ac7f231b8825..cc9513840351 100644
> > --- a/include/linux/backing-dev.h
> > +++ b/include/linux/backing-dev.h
> > @@ -153,6 +153,13 @@ static inline int wb_congested(struct bdi_writeback *wb, int cong_bits)
> >  	return wb->congested & cong_bits;
> >  }
> >  
> > +/* NOTE congestion_wait() and wait_iff_congested() are
> > + * largely useless except as documentation.
> > + * congestion_wait() will (almost) always wait for the given timeout.
> > + * wait_iff_congested() will (almost) never wait, but will call
> > + * cond_resched().
> > + * Were possible an alternative waiting strategy should be found.
> > + */
> >  long congestion_wait(int sync, long timeout);
> >  long wait_iff_congested(int sync, long timeout);
> >  
> > diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> > index 4a9d4e27d0d9..53472ab38796 100644
> > --- a/mm/backing-dev.c
> > +++ b/mm/backing-dev.c
> > @@ -1023,6 +1023,11 @@ EXPORT_SYMBOL(set_bdi_congested);
> >   * Waits for up to @timeout jiffies for a backing_dev (any backing_dev) to exit
> >   * write congestion.  If no backing_devs are congested then just wait for the
> >   * next write to be completed.
> > + *
> > + * NOTE: in the current implementation, hardly any backing_devs are ever
> > + * marked as congested, and write-completion is rarely reported (see calls
> > + * to clear_bdi_congested).  So this should not be assumed to ever wake before
> > + * the timeout.
> >   */
> >  long congestion_wait(int sync, long timeout)
> >  {
> > @@ -1054,6 +1059,10 @@ EXPORT_SYMBOL(congestion_wait);
> >   * The return value is 0 if the sleep is for the full timeout. Otherwise,
> >   * it is the number of jiffies that were still remaining when the function
> >   * returned. return_value == timeout implies the function did not sleep.
> > + *
> > + * NOTE: in the current implementation, hardly any backing_devs are ever
> > + * marked as congested, and write-completion is rarely reported (see calls
> > + * to clear_bdi_congested).  So this should not be assumed to sleep at all.
> >   */
> >  long wait_iff_congested(int sync, long timeout)
> >  {
> > 
> 
> -- 
> Michal Hocko
> SUSE Labs
> 
>