Re: [RESEND PATCH v2] modules: wait do_free_init correctly

From: Luis Chamberlain <mcgrof@kernel.org>
To: Changbin Du <changbin.du@huawei.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-modules@vger.kernel.org, linux-kernel@vger.kernel.org,
	Xiaoyi Su <suxiaoyi@huawei.com>
Subject: Re: [RESEND PATCH v2] modules: wait do_free_init correctly
Date: Tue, 30 Jan 2024 06:21:03 -0800	[thread overview]
Message-ID: <ZbkFz25DtYuhkIEj@bombadil.infradead.org> (raw)
In-Reply-To: <20240130014038.mawqxwoc34v6hztb@M910t>

On Tue, Jan 30, 2024 at 09:40:38AM +0800, Changbin Du wrote:
> On Mon, Jan 29, 2024 at 09:53:58AM -0800, Luis Chamberlain wrote:
> > On Mon, Jan 29, 2024 at 10:03:04AM +0800, Changbin Du wrote:
> > > The commit 1a7b7d922081 ("modules: Use vmalloc special flag") moves
> > > do_free_init() into a global workqueue instead of call_rcu(). So now
> > > rcu_barrier() can not ensure that do_free_init has completed. We should
> > > wait it via flush_work().
> > > 
> > > Without this fix, we still could encounter false positive reports in
> > > W+X checking, and rcu synchronization is unnecessary.
> > 
> > You didn't answer my question, which should be documented in the commit log.
> > 
> > Does this mean we never freed modules init because of this? If so then
> > your commit log should clearly explain that. It should also explain that
> > if true (you have to verify) then it means we were no longer saving
> > the memory we wished to save, and that is important for distributions
> > which do want to save anything on memory. You may want to do a general
> > estimate on how much that means these days on any desktop / server.
>
> Actually, I have explained it in commit msg. It's not about saving memory. The
> synchronization here is just to ensure the module init's been freed before
> doing W+X checking. The problem is that the current implementation is wrong,
> rcu_barrier() cannot guarantee that. So we can encounter false positive reports.
> But anyway, the module init will be freed, and it's just a timing related issue.

Your desciption here is better than the commit log.

  Luis