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SubjectRe: [PATCH] mm/damon: simplify stop mechanism
Date
Hello Changbin,

On Tue, 26 Oct 2021 23:30:33 +0800 Changbin Du <changbin.du@gmail.com> 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@amazon.de/

>
> 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@gmail.com>
> ---
> 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

[...]

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