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SubjectRe: [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)
>> +{
>> + 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);


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