linux-kernel - Re: [RESEND PATCH v2] modules: wait do_free

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Message-ID: <20240130014038.mawqxwoc34v6hztb@M910t>
Date: Tue, 30 Jan 2024 09:40:38 +0800
From: Changbin Du <changbin.du@...wei.com>
To: Luis Chamberlain <mcgrof@...nel.org>
CC: Changbin Du <changbin.du@...wei.com>, Andrew Morton
	<akpm@...ux-foundation.org>, <linux-modules@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Xiaoyi Su <suxiaoyi@...wei.com>
Subject: Re: [RESEND PATCH v2] modules: wait do_free_init correctly

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.

>   Luis

-- 
Cheers,
Changbin Du