linux-kernel - Re: [PATCH v4 3/3] mm/oom_kill: allow process_mrelease to run under mmap

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Date:   Thu, 9 Dec 2021 09:59:03 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, rientjes@...gle.com,
        willy@...radead.org, hannes@...xchg.org, guro@...com,
        riel@...riel.com, minchan@...nel.org, kirill@...temov.name,
        aarcange@...hat.com, christian@...uner.io, hch@...radead.org,
        oleg@...hat.com, david@...hat.com, jannh@...gle.com,
        shakeelb@...gle.com, luto@...nel.org, christian.brauner@...ntu.com,
        fweimer@...hat.com, jengelh@...i.de, timmurray@...gle.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v4 3/3] mm/oom_kill: allow process_mrelease to run under
 mmap_lock protection

On Wed 08-12-21 13:22:11, Suren Baghdasaryan wrote:
> With exit_mmap holding mmap_write_lock during free_pgtables call,
> process_mrelease does not need to elevate mm->mm_users in order to
> prevent exit_mmap from destrying pagetables while __oom_reap_task_mm
> is walking the VMA tree. The change prevents process_mrelease from
> calling the last mmput, which can lead to waiting for IO completion
> in exit_aio.
> 
> Fixes: 337546e83fc7 ("mm/oom_kill.c: prevent a race between process_mrelease and exit_mmap")

I am not sure I have brought this up already but I do not think Fixes
tag is a good fit. 337546e83fc7 is a correct way to handle the race. It
is just slightly less optimal than this fix.

> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>

Acked-by: Michal Hocko <mhocko@...e.com>

Thanks!
> ---
>  mm/oom_kill.c | 27 +++++++++++++++------------
>  1 file changed, 15 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 1ddabefcfb5a..67780386f478 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -1169,15 +1169,15 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
>  		goto put_task;
>  	}
>  
> -	if (mmget_not_zero(p->mm)) {
> -		mm = p->mm;
> -		if (task_will_free_mem(p))
> -			reap = true;
> -		else {
> -			/* Error only if the work has not been done already */
> -			if (!test_bit(MMF_OOM_SKIP, &mm->flags))
> -				ret = -EINVAL;
> -		}
> +	mm = p->mm;
> +	mmgrab(mm);
> +
> +	if (task_will_free_mem(p))
> +		reap = true;
> +	else {
> +		/* Error only if the work has not been done already */
> +		if (!test_bit(MMF_OOM_SKIP, &mm->flags))
> +			ret = -EINVAL;
>  	}
>  	task_unlock(p);
>  
> @@ -1188,13 +1188,16 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
>  		ret = -EINTR;
>  		goto drop_mm;
>  	}
> -	if (!__oom_reap_task_mm(mm))
> +	/*
> +	 * Check MMF_OOM_SKIP again under mmap_read_lock protection to ensure
> +	 * possible change in exit_mmap is seen
> +	 */
> +	if (!test_bit(MMF_OOM_SKIP, &mm->flags) && !__oom_reap_task_mm(mm))
>  		ret = -EAGAIN;
>  	mmap_read_unlock(mm);
>  
>  drop_mm:
> -	if (mm)
> -		mmput(mm);
> +	mmdrop(mm);
>  put_task:
>  	put_task_struct(task);
>  	return ret;
> -- 
> 2.34.1.400.ga245620fadb-goog

-- 
Michal Hocko
SUSE Labs