linux-kernel - Re: [PATCH] mm/mremap: avoid unneeded do

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Date:   Fri, 1 Apr 2022 12:38:53 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Miaohe Lin <linmiaohe@...wei.com>, akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/mremap: avoid unneeded do_munmap call

On 01.04.22 10:10, Miaohe Lin wrote:
> When old_len == new_len, do_munmap will return -EINVAL due to len == 0.
> This errno will be simply ignored because of old_len != new_len check.
> So it is unnecessary to call do_munmap when old_len == new_len because
> nothing is actually done.
> 
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  mm/mremap.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/mremap.c b/mm/mremap.c
> index e776d4c2345c..dd966621a056 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -817,9 +817,9 @@ static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
>  			goto out;
>  	}
>  
> -	if (old_len >= new_len) {
> +	if (old_len > new_len) {
>  		ret = do_munmap(mm, addr+new_len, old_len - new_len, uf_unmap);
> -		if (ret && old_len != new_len)
> +		if (ret)
>  			goto out;
>  		old_len = new_len;
>  	}

I remember stumbling over that myself a year ago or so but dig not
deeper. But indeed, both variants (mmu, nommu) return -EINVAL in case
len (old_len - new_len) == 0.

Maybe that used to be different before ecc1a8993751 ("do_mremap()
untangling, part 2"), but it doesn't look like it.

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Thanks,

David / dhildenb