Hello Changbin, On Tue, 26 Oct 2021 23:30:33 +0800 Changbin Du <changbin.du@xxxxxxxxx> wrote: > An kernel thread can exit gracefully with kthread_stop(). So we don't need a > new flag 'kdamond_stop'. And to make sure the task struct is not freed when > accessing it, get task struct on start and put it on stop. We previously considered using kthread_stop() here. However, we resulted in current code because kdamond can be self-terminated when all target processes are invalid[1]. Seems this patch is also not fully prepared for the self-termination case. I left some comments below. [1] https://lore.kernel.org/linux-mm/20210624102623.24563-1-sjpark@xxxxxxxxx/ > > And since the return value of 'before_terminate' callback is never used, > we make it have no return value. This looks nice to me. Could you please send this again as a separate patch? > > Signed-off-by: Changbin Du <changbin.du@xxxxxxxxx> > --- > include/linux/damon.h | 3 +-- > mm/damon/core.c | 59 +++++++++++++------------------------------ > mm/damon/dbgfs.c | 5 ++-- > 3 files changed, 20 insertions(+), 47 deletions(-) > > diff --git a/include/linux/damon.h b/include/linux/damon.h > index a14b3cc54cab..041966786270 100644 > --- a/include/linux/damon.h > +++ b/include/linux/damon.h [...] > --- a/mm/damon/core.c > +++ b/mm/damon/core.c [...] > @@ -1069,7 +1048,7 @@ static int kdamond_fn(void *data) > sz_limit); > if (ctx->callback.after_aggregation && > ctx->callback.after_aggregation(ctx)) > - set_kdamond_stop(ctx); > + done = true; > kdamond_apply_schemes(ctx); > kdamond_reset_aggregated(ctx); > kdamond_split_regions(ctx); > @@ -1088,16 +1067,12 @@ static int kdamond_fn(void *data) > damon_destroy_region(r, t); > } > > - if (ctx->callback.before_terminate && > - ctx->callback.before_terminate(ctx)) > - set_kdamond_stop(ctx); > + if (ctx->callback.before_terminate) > + ctx->callback.before_terminate(ctx); > if (ctx->primitive.cleanup) > ctx->primitive.cleanup(ctx); > > pr_debug("kdamond (%d) finishes\n", current->pid); > - mutex_lock(&ctx->kdamond_lock); > - ctx->kdamond = NULL; > - mutex_unlock(&ctx->kdamond_lock); When kdamond is self-terminating, ctx->kdamond will not be nullfified. As a result, this patch can introduce some errors like below: # cd /sys/kernel/debug/damon # sleep 60 & [1] 1926 # echo $(pidof sleep) > target_ids # echo on > monitor_on # cat monitor_on on # # after 60 seconds, sleep finishes and kdamond is self-terminated # cat monitor_on off # echo 42 > target_ids bash: echo: write error: Device or resource busy If we simply restore the nullification here with the mutex locking, we would result in a deadlock because __damon_stop() calls kthread_stop() while holding ctx->kdamond_lock. Also, the reference count of ctx->kdamond, which increased by __damon_start(), would not be decreased in the case. If I'm missing something, please let me know. Thanks, SJ [...]
