linux-kernel - Re: [v2 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm

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Date:   Tue, 27 Mar 2018 08:29:39 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     adobriyan@...il.com, willy@...radead.org, mguzik@...hat.com,
        gorcunov@...nvz.org, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH] mm: introduce arg_lock to protect arg_start|end and
 env_start|end in mm_struct

On Tue 27-03-18 02:20:39, Yang Shi wrote:
[...]
The patch looks reasonable to me. Maybe it would be better to be more
explicit about the purpose of the patch. As others noticed, this alone
wouldn't solve the mmap_sem contention issues. I _think_ that if you
were more explicit about the mmap_sem abuse it would trigger less
questions.

I have just one more question. Now that you are touching this area,
would you be willing to remove the following ugliness?

> diff --git a/kernel/sys.c b/kernel/sys.c
> index f2289de..17bddd2 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -1959,7 +1959,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
>  			return error;
>  	}
>  
> -	down_write(&mm->mmap_sem);
> +	down_read(&mm->mmap_sem);

Why do we need to hold mmap_sem here and call find_vma, when only
PR_SET_MM_ENV_END: is consuming it? I guess we can replace it wit the
new lock and take the mmap_sem only for PR_SET_MM_ENV_END.

Thanks!
-- 
Michal Hocko
SUSE Labs