linux-kernel - Re: [PATCH] mm: Unsigned 'nr_pages' always larger than zero

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Date:   Wed, 4 Sep 2019 13:24:58 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     zhong jiang <zhongjiang@...wei.com>, akpm@...ux-foundation.org,
        mhocko@...nel.org
Cc:     anshuman.khandual@....com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Ira Weiny <ira.weiny@...el.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: Re: [PATCH] mm: Unsigned 'nr_pages' always larger than zero

On 9/4/19 12:26 PM, zhong jiang wrote:
> With the help of unsigned_lesser_than_zero.cocci. Unsigned 'nr_pages"'
> compare with zero. And __get_user_pages_locked will return an long value.
> Hence, Convert the long to compare with zero is feasible.

It would be nicer if the parameter nr_pages was long again instead of unsigned
long (note there are two variants of the function, so both should be changed).

> Signed-off-by: zhong jiang <zhongjiang@...wei.com>

Fixes: 932f4a630a69 ("mm/gup: replace get_user_pages_longterm() with FOLL_LONGTERM")

(which changed long to unsigned long)

AFAICS... stable shouldn't be needed as the only "risk" is that we goto
check_again even when we fail, which should be harmless.

Vlastimil

> ---
>  mm/gup.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 23a9f9c..956d5a1 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1508,7 +1508,7 @@ static long check_and_migrate_cma_pages(struct task_struct *tsk,
>  						   pages, vmas, NULL,
>  						   gup_flags);
>  
> -		if ((nr_pages > 0) && migrate_allow) {
> +		if (((long)nr_pages > 0) && migrate_allow) {
>  			drain_allow = true;
>  			goto check_again;
>  		}
>