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Date: Sun, 5 Jul 2026 15:10:17 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Hajime Tazaki <thehajime@gmail.com>
Cc: liam@infradead.org, ljs@kernel.org, vbabka@kernel.org, jannh@google.com,
 pfalcato@suse.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: nommu: add sysctl_max_map_count() check for
 do_mmap()
Message-Id: <20260705151017.289ed34630b503cdc1def407@linux-foundation.org>
In-Reply-To: <20260702012830.667205-1-thehajime@gmail.com>
References: <20260702012830.667205-1-thehajime@gmail.com>
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On Thu,  2 Jul 2026 10:28:30 +0900 Hajime Tazaki <thehajime@gmail.com> wrote:

> The sysctl variable vm.max_map_count (sysctl_max_map_count) is not
> expose under !MMU case but used it wit the default vaule
> DEFAULT_MAX_MAP_COUNT as a limit of count.

This sounds like an oversight.  I see no reason why nommu users cannot
alter max_map_count.

> This is currently used when
> a vma entry is split into two chunks (split_vma()) but not used when
> allocated (do_mmap()).  As a result, even if users request a large
> number of allocate memory, it will keep allocating until OOM happens.
> 
> This commit introduces a check at the begining of do_mmap in nommu.c to
> prevent this situation.
> 
> ...
>
> --- a/mm/nommu.c
> +++ b/mm/nommu.c
> @@ -1035,6 +1035,9 @@ unsigned long do_mmap(struct file *file,
>  	if (ret < 0)
>  		return ret;
>  
> +	if (current->mm->map_count >= get_sysctl_max_map_count())
> +		return -ENOMEM;
> +
>  	/* we ignore the address hint */
>  	addr = 0;
>  	len = PAGE_ALIGN(len);

Seems sensible, thanks.  That's what the !NOMMU code does.