Re: [PATCH] memcg: convert threshold to bytes

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 7 Oct 2015 09:30:02 +0200 Michal Hocko <mhocko@kernel.org> wrote:

> On Tue 06-10-15 12:22:25, Andrew Morton wrote:
> > On Tue, 6 Oct 2015 19:01:23 +0200 Michal Hocko <mhocko@kernel.org> wrote:
> > 
> > > On Mon 05-10-15 14:44:22, Shaohua Li wrote:
> > > > The page_counter_memparse() returns pages for the threshold, while
> > > > mem_cgroup_usage() returns bytes for memory usage. Convert the threshold
> > > > to bytes.
> > > > 
> > > > Looks a regression introduced by 3e32cb2e0a12b69150
> > > 
> > > Yes. This suggests
> > > Cc: stable # 3.19+
> > 
> > But it's been this way for 2 years and nobody noticed it.  How come?
> 
> Maybe we do not have that many users of this API with newer kernels.

Either it's zero or all the users have worked around this bug.

> > Or at least, nobody reported it.  Maybe people *have* noticed it, and
> > adjusted their userspace appropriately.  In which case this patch will
> > cause breakage.
> 
> I dunno, I would rather have it fixed than keep bug to bug compatibility
> because they would eventually move to a newer kernel one day when they
> see the "breakage" anyway.

They'd only see breakage if we fixed this in the newer kernel.

We could just change the docs and leave it as-is.  That it is called
"usage_in_bytes" makes that a bit awkward.

A bit of googling indicates that people are using usage_in_bytes.  A
few.  All the discussions I found clearly predate this bug.

So did people just stop using this?  Is there some alternative way of
getting the same info?

Why does memcg_write_event_control() says "DO NOT USE IN NEW FILES"
and "DO NOT ADD NEW FILES"?

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