linux-kernel - Re: [PATCH 3/6] lib/scatterlist: Do not leak pages when high-order allocation fails

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Message-ID: <1520439389.2890.16.camel@wdc.com>
Date:   Wed, 7 Mar 2018 16:16:30 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tursulin@...ulin.net" <tursulin@...ulin.net>
CC:     "tvrtko.ursulin@...el.com" <tvrtko.ursulin@...el.com>,
        "hare@...e.com" <hare@...e.com>,
        "jthumshirn@...e.de" <jthumshirn@...e.de>,
        "axboe@...nel.dk" <axboe@...nel.dk>
Subject: Re: [PATCH 3/6] lib/scatterlist: Do not leak pages when high-order
 allocation fails

On Wed, 2018-03-07 at 12:47 +0000, Tvrtko Ursulin wrote:
> diff --git a/lib/scatterlist.c b/lib/scatterlist.c
> index 9884be50a2c0..e13a759c5c49 100644
> --- a/lib/scatterlist.c
> +++ b/lib/scatterlist.c
> @@ -493,7 +493,7 @@ struct scatterlist *sgl_alloc_order(unsigned long length, unsigned int order,
>  {
>  	unsigned int chunk_len = PAGE_SIZE << order;
>  	struct scatterlist *sgl, *sg;
> -	unsigned int nent;
> +	unsigned int nent, i;
>  
>  	nent = round_up(length, chunk_len) >> (PAGE_SHIFT + order);
>  
> @@ -517,11 +517,12 @@ struct scatterlist *sgl_alloc_order(unsigned long length, unsigned int order,
>  
>  	sg_init_table(sgl, nent);
>  	sg = sgl;
> +	i = 0;
>  	while (length) {
>  		struct page *page = alloc_pages(gfp, order);
>  
>  		if (!page) {
> -			sgl_free(sgl);
> +			sgl_free_n_order(sgl, i, order);
>  			return NULL;
>  		}
>  
> @@ -529,6 +530,7 @@ struct scatterlist *sgl_alloc_order(unsigned long length, unsigned int order,
>  		sg_set_page(sg, page, chunk_len, 0);
>  		length -= chunk_len;
>  		sg = sg_next(sg);
> +		i++;
>  	}

Since the entire sg-list is zero-initialized before this loop starts, since
the sg-list is not chained onto another sg-list before this function returns
and since sgl_free_n_order() checks whether or not each page pointer is NULL
before freeing it I think we don't need the new loop variable 'i' and that
we can call sgl_free_order() instead of sgl_free_n_order().

Bart.