linux-kernel - Re: [PATCH rdma-next 3/4] lib/scatterlist: Add support in dynamic allocation of SG table from pages

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Message-ID: <20200907072921.GC19875@lst.de>
Date:   Mon, 7 Sep 2020 09:29:21 +0200
From:   Christoph Hellwig <hch@....de>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     Christoph Hellwig <hch@....de>, Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Maor Gottlieb <maorg@...dia.com>, linux-kernel@...r.kernel.org,
        linux-rdma@...r.kernel.org
Subject: Re: [PATCH rdma-next 3/4] lib/scatterlist: Add support in dynamic
 allocation of SG table from pages

On Thu, Sep 03, 2020 at 03:18:52PM +0300, Leon Romanovsky wrote:
> +struct sg_append {
> +	struct scatterlist *prv; /* Previous entry to append */
> +	unsigned int left_pages; /* Left pages to add to table */
> +};

I don't really see the point in this structure.   Either pass it as
two separate arguments, or switch sg_alloc_table_append and the
internal helper to pass all arguments as a struct.

> + *    A user may provide an offset at a start and a size of valid data in a buffer
> + *    specified by the page array. A user may provide @append to chain pages to

This adds a few pointles > 80 char lines.

> +struct scatterlist *
> +sg_alloc_table_append(struct sg_table *sgt, struct page **pages,
> +		      unsigned int n_pages, unsigned int offset,
> +		      unsigned long size, unsigned int max_segment,
> +		      gfp_t gfp_mask, struct sg_append *append)
> +{
> +#ifdef CONFIG_ARCH_NO_SG_CHAIN
> +	if (append->left_pages)
> +		return ERR_PTR(-EOPNOTSUPP);
> +#endif

Which makes this API entirely useless for !CONFIG_ARCH_NO_SG_CHAIN,
doesn't it?  Wouldn't it make more sense to not provide it for that
case and add an explicitl dependency in the callers?

> +	return alloc_from_pages_common(sgt, pages, n_pages, offset, size,
> +				       max_segment, gfp_mask, append);

And if we somehow manage to sort that out we can merge
sg_alloc_table_append and alloc_from_pages_common, reducing the amount
of wrappers that just make it too hard to follow the code.

> +EXPORT_SYMBOL(sg_alloc_table_append);

EXPORT_SYMBOL_GPL, please.