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

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Date:   Mon, 7 Sep 2020 15:44:08 +0300
From:   Maor Gottlieb <maorg@...dia.com>
To:     Christoph Hellwig <hch@....de>, Leon Romanovsky <leon@...nel.org>
CC:     Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...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 9/7/2020 10:29 AM, Christoph Hellwig wrote:
> 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.

I did it to avoid more than 8 arguments of this function, will change it 
to be 9 if it's fine for you.
>
>> + *    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.

Will fix.
>
>> +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?

Current implementation allow us to support small memory registration 
which not require chaining. I am not aware which archs has the SG_CHAIN 
support and I don't want to break it so I can't add it to as dependency 
to the Kconfig. Another option is to do the logic in the caller, but it 
isn't clean.

>
>> +	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.

Sure