Re: [PATCH 1/2] fs/dcache: Track & report number of negative dentries

[Matthew Wilcox <willy@infradead.org>]

On Wed, Aug 29, 2018 at 10:11:53AM +1000, Dave Chinner wrote:
> > +++ b/Documentation/sysctl/fs.txt
> > @@ -63,19 +63,26 @@ struct {
> >          int nr_unused;
> >          int age_limit;         /* age in seconds */
> >          int want_pages;        /* pages requested by system */
> > -        int dummy[2];
> > +        int nr_negative;       /* # of unused negative dentries */
> > +        int dummy;
> >  } dentry_stat = {0, 0, 45, 0,};
> 
> That's not a backwards compatible ABI change. Those dummy fields
> used to represent some metric we no longer calculate, and there are
> probably still monitoring apps out there that think they still have
> the old meaning. i.e. they are still visible to userspace:

I believe you are incorrect.  dentry_stat was introduced in 2.1.60 with
this hunk:

+struct {
+       int nr_dentry;
+       int nr_unused;
+       int age_limit;          /* age in seconds */
+       int want_pages;         /* pages requested by system */
+       int dummy[2];
+} dentry_stat = {0, 0, 45, 0,};
+

Looking through the rest of the dentry_stat changes in the 2.1.60 release,
it's not replacing anything, it's adding new information.