linux-kernel - Re: [RFC PATCH 05/11] mm/mempolicy: modify set_mempolicy_home

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Date:   Tue, 28 Nov 2023 15:07:38 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Gregory Price <gourry.memverge@...il.com>
Cc:     linux-mm@...ck.org, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, arnd@...db.de, tglx@...utronix.de,
        luto@...nel.org, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        tj@...nel.org, ying.huang@...el.com,
        Gregory Price <gregory.price@...verge.com>
Subject: Re: [RFC PATCH 05/11] mm/mempolicy: modify set_mempolicy_home_node
 to take a task argument

On Wed 22-11-23 16:11:54, Gregory Price wrote:
[...]
> +
> +	/*
> +	 * Behavior when task == current allows a task modifying itself
> +	 * to bypass the check in get_task_mm and acquire the mm directly
> +	 */
> +	if (task == current) {
> +		mm = task->mm;
> +		mmget(mm);
> +	} else
> +		mm = get_task_mm(task);

Similar question as in the previous patch. When is a kthread legit to do
manipulate borrowed mm memory policy?

> +
> +	if (!mm)
> +		return -ENODEV;

Is this an expected error? No mm means task dying and mm has been
already released. Should we simply return ESRCH wouldbe a better error
code IMO. This should never happen for the current task so it sould be
safe to add.

-- 
Michal Hocko
SUSE Labs