Re: [PATCH] mm: vmscan: handle any negative return value fromscan_objects

From: Oskar Andero
Date: Thu May 16 2013 - 08:28:00 EST

On 13:52 Thu 16 May , Dave Chinner wrote:
> On Thu, May 16, 2013 at 10:42:16AM +0200, Oskar Andero wrote:
> > The shrinkers must return -1 to indicate that it is busy. Instead, treat
> > any negative value as busy.
> Why? The API defines return condition for aborting a scan and gives
> a specific value for doing that. i.e. explain why should change the
> API to over-specify the 'abort scan" return value like this.

As I pointed out earlier, looking in to the code (from master):
if (shrink_ret == -1)
if (shrink_ret < nr_before)
ret += nr_before - shrink_ret;

This piece of code lacks a sanity check and will only function if shrink_ret
is either greater than zero or exactly -1. If shrink_ret is e.g. -2 this will
lead to undefined behaviour.

> FWIW, using "any" negative number for "abort scan" is a bad API
> design decision. It means that in future we can't introduce
> different negative return values in the API if we have a new to.
> i.e. each specific negative return value needs to have the potential
> for defining a different behaviour.

An alternative to my patch would be to add:
if (shrink_ret < -1)
/* handle illegal return code in some way */

> So if any change needs to be made, it is to change the -1 return
> value to an enum and have the shrinkers return that enum when they
> want an abort.

I am all for an enum, but I still believe we should handle the case where
the shrinkers return something wicked.

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