Re: (sashiko review) [RFC PATCH 1/2] mm/damon/stat: add a parameter for reading kdamond pid

[SeongJae Park <sj@kernel.org>]

TL; DR: Sashiko found a bug of this patch; I will fix it in the next revision.
Sashiko also found one issue in the existing code; I will work on it
separately.

Forwarding full sashiko review in a reply format with my inline comments below,
for sharing details of my view and doing followup discussions via mails if
needed.

> # review url: https://sashiko.dev/#/patchset/20260414045002.88110-2-sj@kernel.org
> # start of sashiko.dev inline review
> commit fc6c9ec12df0df57fcce5c90c7a0604cddb1cc78
> Author: SeongJae Park <sj@kernel.org>
> 
> mm/damon/stat: add a parameter for reading kdamond pid
> 
> This commit exposes the PID of the DAMON_STAT kdamond via a new read-only
> module parameter, kdamond_pid. It intends to set this to -1 when DAMON_STAT
> is disabled.
> 
> > diff --git a/mm/damon/stat.c b/mm/damon/stat.c
> > index 99ba346f9e325..21fa04d95eeac 100644
> > --- a/mm/damon/stat.c
> > +++ b/mm/damon/stat.c
> [ ... ]
> > @@ -260,6 +269,9 @@ static int damon_stat_start(void)
> >  		damon_stat_context = NULL;
> >  		return err;
> >  	}
> > +	kdamond_pid = damon_kdamond_pid(damon_stat_context);
> > +	if (kdamond_pid < 0)
> > +		return kdamond_pid;
> 
> If damon_kdamond_pid() returns an error code, doesn't this leave the global
> kdamond_pid set to that negative error code instead of -1?
> 
> Also, if we take this early return path, does this code leak the
> damon_stat_context allocation? The error path just above it correctly
> calls damon_destroy_ctx(damon_stat_context) and sets it to NULL.

Nice catch.  I will fix this like below in the next revision.

'''
--- a/mm/damon/stat.c
+++ b/mm/damon/stat.c
@@ -223,8 +223,12 @@ static int damon_stat_start(void)
                return err;
        }
        kdamond_pid = damon_kdamond_pid(damon_stat_context);
-       if (kdamond_pid < 0)
-               return kdamond_pid;
+       if (kdamond_pid < 0) {
+               err = kdamond_pid;
+               kdamond_pid = -1;
+               damon_destroy_ctx(damon_stat_context);
+               damon_stat_context = NULL;
+               return err;

        damon_stat_last_refresh_jiffies = jiffies;
        call_control.data = damon_stat_context;
'''

> 
> >  
> >  	damon_stat_last_refresh_jiffies = jiffies;
> >  	call_control.data = damon_stat_context;
> >  	return damon_call(damon_stat_context, &call_control);
> >  }
> 
> If damon_call() fails here and returns an error, the caller will handle it
> by setting enabled to false. Does this leave kdamond_pid holding a stale
> positive PID instead of being reset to -1?

Nice catch, but not this patch's fault.  I will separately work on it.

> 
> > @@ -269,6 +281,7 @@ static int damon_stat_start(void)
> >  static void damon_stat_stop(void)
> >  {
> >  	damon_stop(&damon_stat_context, 1);
> > +	kdamond_pid = -1;
> >  	damon_destroy_ctx(damon_stat_context);
> >  	damon_stat_context = NULL;
> >  }
> 
> 
> # end of sashiko.dev inline review
> # review url: https://sashiko.dev/#/patchset/20260414045002.88110-2-sj@kernel.org


Thanks,
SJ

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