linux-kernel - Re: [PATCH 4/5] mm: gup: fix potential pgmap refcnt leak in __gup_device

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Date:   Sat, 7 Aug 2021 11:41:12 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     <imbrenda@...ux.ibm.com>, <kirill.shutemov@...ux.intel.com>,
        <jack@...e.cz>, <jhubbard@...dia.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/5] mm: gup: fix potential pgmap refcnt leak in
 __gup_device_huge()

On Sat, 7 Aug 2021 17:36:19 +0800 Miaohe Lin <linmiaohe@...wei.com> wrote:

> When failed to try_grab_page, put_dev_pagemap() is missed. So pgmap
> refcnt will leak in this case. Also we remove the check for pgmap
> against NULL as it's also checked inside the put_dev_pagemap().
> 
> ...
>
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2253,14 +2253,14 @@ static int __gup_device_huge(unsigned long pfn, unsigned long addr,
>  		pages[*nr] = page;
>  		if (unlikely(!try_grab_page(page, flags))) {
>  			undo_dev_pagemap(nr, nr_start, flags, pages);
> +			put_dev_pagemap(pgmap);
>  			return 0;
>  		}
>  		(*nr)++;
>  		pfn++;
>  	} while (addr += PAGE_SIZE, addr != end);
>  
> -	if (pgmap)
> -		put_dev_pagemap(pgmap);
> +	put_dev_pagemap(pgmap);
>  	return 1;
>  }

We can simplify further, and remove the troublesome multiple return points?

--- a/mm/gup.c~mm-gup-fix-potential-pgmap-refcnt-leak-in-__gup_device_huge-fix
+++ a/mm/gup.c
@@ -2247,14 +2247,13 @@ static int __gup_device_huge(unsigned lo
 		pgmap = get_dev_pagemap(pfn, pgmap);
 		if (unlikely(!pgmap)) {
 			undo_dev_pagemap(nr, nr_start, flags, pages);
-			return 0;
+			break;
 		}
 		SetPageReferenced(page);
 		pages[*nr] = page;
 		if (unlikely(!try_grab_page(page, flags))) {
 			undo_dev_pagemap(nr, nr_start, flags, pages);
-			put_dev_pagemap(pgmap);
-			return 0;
+			break;
 		}
 		(*nr)++;
 		pfn++;
_