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

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Message-ID: <29d686c3-9407-e953-42f4-a4224b9333e7@suse.cz>
Date:   Wed, 4 Sep 2019 22:40:22 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     zhong jiang <zhongjiang@...wei.com>, mhocko@...nel.org,
        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 8:48 PM, Andrew Morton wrote:
> On Wed, 4 Sep 2019 13:24:58 +0200 Vlastimil Babka <vbabka@...e.cz> wrote:
> 
>> 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).
> 
> nr_pages should be unsigned - it's a count of pages!

Hm right, I thought check_and_migrate_cma_pages() could be already
called with negative nr_pages from __gup_longterm_locked(), but I missed
there's a if (rc < 0) goto out before that.

> The bug is that __get_user_pages_locked() returns a signed long which
> can be a -ve errno.
> 
> I think it's best if __get_user_pages_locked() is to get itself a new

You mean check_and_migrate_cma_pages()

> local with the same type as its return value.  Something like:

Agreed.

> --- a/mm/gup.c~a
> +++ a/mm/gup.c
> @@ -1450,6 +1450,7 @@ static long check_and_migrate_cma_pages(
>  	bool drain_allow = true;
>  	bool migrate_allow = true;
>  	LIST_HEAD(cma_page_list);
> +	long ret;

Should be initialized to nr_pages in case we don't go via "if
(!list_empty(&cma_page_list))" at all.

>  
>  check_again:
>  	for (i = 0; i < nr_pages;) {
> @@ -1511,17 +1512,18 @@ check_again:
>  		 * again migrating any new CMA pages which we failed to isolate
>  		 * earlier.
>  		 */
> -		nr_pages = __get_user_pages_locked(tsk, mm, start, nr_pages,
> +		ret = __get_user_pages_locked(tsk, mm, start, nr_pages,
>  						   pages, vmas, NULL,
>  						   gup_flags);
>  
> -		if ((nr_pages > 0) && migrate_allow) {
> +		nr_pages = ret;
> +		if (ret > 0 && migrate_allow) {
>  			drain_allow = true;
>  			goto check_again;
>  		}
>  	}
>  
> -	return nr_pages;
> +	return ret;
>  }
>  #else
>  static long check_and_migrate_cma_pages(struct task_struct *tsk,
> 
>