linux-kernel - Re: [PATCH v5 1/4] sgl_alloc_order: remove 4 GiB limit, sgl

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Date:   Thu, 7 Jan 2021 13:44:10 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Douglas Gilbert <dgilbert@...erlog.com>
Cc:     linux-scsi@...r.kernel.org, linux-block@...r.kernel.org,
        target-devel@...r.kernel.org, linux-rdma@...r.kernel.org,
        linux-kernel@...r.kernel.org, martin.petersen@...cle.com,
        jejb@...ux.vnet.ibm.com, bostroesser@...il.com, bvanassche@....org,
        ddiss@...e.de
Subject: Re: [PATCH v5 1/4] sgl_alloc_order: remove 4 GiB limit, sgl_free()
 warning

On Mon, Dec 28, 2020 at 06:49:52PM -0500, Douglas Gilbert wrote:
> diff --git a/lib/scatterlist.c b/lib/scatterlist.c
> index a59778946404..4986545beef9 100644
> +++ b/lib/scatterlist.c
> @@ -554,13 +554,15 @@ EXPORT_SYMBOL(sg_alloc_table_from_pages);
>  #ifdef CONFIG_SGL_ALLOC
>  
>  /**
> - * sgl_alloc_order - allocate a scatterlist and its pages
> + * sgl_alloc_order - allocate a scatterlist with equally sized elements
>   * @length: Length in bytes of the scatterlist. Must be at least one
> - * @order: Second argument for alloc_pages()
> + * @order: Second argument for alloc_pages(). Each sgl element size will
> + *	   be (PAGE_SIZE*2^order) bytes
>   * @chainable: Whether or not to allocate an extra element in the scatterlist
> - *	for scatterlist chaining purposes
> + *	       for scatterlist chaining purposes
>   * @gfp: Memory allocation flags
> - * @nent_p: [out] Number of entries in the scatterlist that have pages
> + * @nent_p: [out] Number of entries in the scatterlist that have pages.
> + *		  Ignored if NULL is given.
>   *
>   * Returns: A pointer to an initialized scatterlist or %NULL upon failure.
>   */
> @@ -574,8 +576,8 @@ struct scatterlist *sgl_alloc_order(unsigned long long length,
>  	u32 elem_len;
>  
>  	nent = round_up(length, PAGE_SIZE << order) >> (PAGE_SHIFT + order);
> -	/* Check for integer overflow */
> -	if (length > (nent << (PAGE_SHIFT + order)))
> +	/* Integer overflow if:  length > nent*2^(PAGE_SHIFT+order) */
> +	if (ilog2(length) > ilog2(nent) + PAGE_SHIFT + order)
>  		return NULL;
>  	nalloc = nent;
>  	if (chainable) {

This is a little bit too tortured now, how about this:

	if (length >> (PAGE_SHIFT + order) >= UINT_MAX)
		return NULL;
	nent = length >> (PAGE_SHIFT + order);
	if (length & ((1ULL << (PAGE_SHIFT + order)) - 1))
		nent++;

	if (chainable) {
		if (check_add_overflow(nent, 1, &nalloc))
			return NULL;
	}
	else
		nalloc = nent;

Jason