Re: [PATCH rdma-next v1 3/4] lib/scatterlist: Add support in dynamic allocation of SG table from pages

Christoph Hellwig
2020-09-15T17:14:25Z

> +#ifndef CONFIG_ARCH_NO_SG_CHAIN
> +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 scatterlist *prv, unsigned int left_pages);
> +#endif

Odd indentation here, we either do two tabs (my preference) or aligned
to the opening brace (what you seem to be doing elsewhere in the series).

> + /* Check if last entry should be keeped for chainning */
> + next_sg = sg_next(prv);
> + if (!sg_is_last(next_sg) || left_npages == 1)
> + return next_sg;
> +
> + ret = sg_alloc_next(table, next_sg,
> + min_t(unsigned long, left_npages,
> + SG_MAX_SINGLE_ALLOC),
> + SG_MAX_SINGLE_ALLOC, gfp_mask);

Do we even need the sg_alloc_next helper added in the last patch,
given that this fairly simple function is the only caller?

> +static struct scatterlist *alloc_from_pages_common(
> + 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 scatterlist *prv, unsigned int left_pages)

Same strange one tab indent as above.

> +#ifndef CONFIG_ARCH_NO_SG_CHAIN
> +/**
> + * sg_alloc_table_append - Allocate and initialize an sg table from
> + * an array of pages
> + * @sgt: The sg table header to use
> + * @pages: Pointer to an array of page pointers
> + * @n_pages: Number of pages in the pages array
> + * @offset: Offset from start of the first page to the start of a buffer
> + * @size: Number of valid bytes in the buffer (after offset)
> + * @max_segment: Maximum size of a scatterlist node in bytes (page aligned)
> + * @gfp_mask: GFP allocation mask
> + * @prv: Last populated sge in sgt
> + * @left_pages: Left pages caller have to set after this call
> + *
> + * Description:
> + * If @prv is NULL, it allocates and initialize an sg table from a list of
> + * pages. Contiguous ranges of the pages are squashed into a single
> + * scatterlist node up to the maximum size specified in @max_segment. 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 last entry in sgt. The returned sg table is released by sg_free_table.
> + *
> + * Returns:
> + * Last SGE in sgt on success, negative error on failure.
> + *
> + * Notes:
> + * If this function returns non-0 (eg failure), the caller must call
> + * sg_free_table() to cleanup any leftover allocations.
> + */
> +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 scatterlist *prv, unsigned int left_pages)

One-tab indent again.

> +{
> + return alloc_from_pages_common(sgt, pages, n_pages, offset, size,
> + max_segment, gfp_mask, prv, left_pages);
> +}
> +EXPORT_SYMBOL_GPL(sg_alloc_table_append);
> +#endif

So there reason I suggested to not provide sg_alloc_table_append
if CONFIG_ARCH_NO_SG_CHAIN was set was to avoid the extra
alloc_from_pages_common helper. It might be better to move to your
run-time check and just make it conditіtional on a non-NULL prv pointer,
which would allow us to merge alloc_from_pages_common into
sg_alloc_table_append. Sorry for leading you down this path.