From: SeongJae Park <sj@kernel.org>
To: Changbin Du <changbin.du@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
SeongJae Park <sj@kernel.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/damon: simplify stop mechanism
Date: Tue, 26 Oct 2021 18:42:03 +0000 [thread overview]
Message-ID: <20211026184203.1541-1-sj@kernel.org> (raw)
In-Reply-To: <20211026153033.11140-1-changbin.du@gmail.com>
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
[...]
next prev parent reply other threads:[~2021-10-26 18:42 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-26 15:30 [PATCH] mm/damon: simplify stop mechanism Changbin Du
2021-10-26 18:42 ` SeongJae Park [this message]
2021-10-27 6:13 ` Changbin Du
2021-10-27 10:51 ` SeongJae Park
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