Re: [Intel-gfx] [PATCH rdma-next v4 1/4] lib/scatterlist: Add support in dynamic allocation of SG table from pages

Jason Gunthorpe
2020-10-02T15:45:04Z

On Sun, Sep 27, 2020 at 09:46:44AM +0300, Leon Romanovsky wrote:
> +struct scatterlist *__sg_alloc_table_from_pages(struct sg_table *sgt,
> + struct page **pages, unsigned int n_pages, unsigned int offset,
> + unsigned long size, unsigned int max_segment,
> + struct scatterlist *prv, unsigned int left_pages,
> + gfp_t gfp_mask)
> {
> - unsigned int chunks, cur_page, seg_len, i;
> + unsigned int chunks, cur_page, seg_len, i, prv_len = 0;
> + struct scatterlist *s = prv;
> + unsigned int table_size;
> + unsigned int tmp_nents;
> int ret;
> - struct scatterlist *s;
>
> if (WARN_ON(!max_segment || offset_in_page(max_segment)))
> - return -EINVAL;
> + return ERR_PTR(-EINVAL);
> + if (IS_ENABLED(CONFIG_ARCH_NO_SG_CHAIN) && prv)
> + return ERR_PTR(-EOPNOTSUPP);
> +
> + tmp_nents = prv ? sgt->nents : 0;
> +
> + if (prv &&
> + page_to_pfn(sg_page(prv)) + (prv->length >> PAGE_SHIFT) ==

This calculation of the end doesn't consider sg->offset

> + page_to_pfn(pages[0]))
> + prv_len = prv->length;
>
> /* compute number of contiguous chunks */
> chunks = 1;
> @@ -410,13 +461,17 @@ int __sg_alloc_table_from_pages(struct sg_table *sgt, struct page **pages,
> }
> }
>
> - ret = sg_alloc_table(sgt, chunks, gfp_mask);
> - if (unlikely(ret))
> - return ret;
> + if (!prv) {
> + /* Only the last allocation could be less than the maximum */
> + table_size = left_pages ? SG_MAX_SINGLE_ALLOC : chunks;
> + ret = sg_alloc_table(sgt, table_size, gfp_mask);
> + if (unlikely(ret))
> + return ERR_PTR(ret);
> + }

This is basically redundant right? Now that get_next_sg() can allocate
SGs it can just build them one by one, no need to preallocate.

Actually all the changes the the allocation seem like overkill, just
allocate a single new array directly in get_next_sg() whenever it
needs.

Something like this:

@@ -365,6 +372,37 @@ int sg_alloc_table(struct sg_table *table, unsigned int nents, gfp_t gfp_mask)
}
EXPORT_SYMBOL(sg_alloc_table);

+static struct scatterlist *get_next_sg(struct sg_table *table,
+ struct scatterlist *cur, unsigned long needed_sges,
+ gfp_t gfp_mask)
+{
+ struct scatterlist *new_sg;
+ unsigned int alloc_size;
+
+ if (cur) {
+ struct scatterlist *next_sg = sg_next(cur);
+
+ /* Check if last entry should be keeped for chainning */
+ if (!sg_is_last(next_sg) || needed_sges == 1)
+ return next_sg;
+ }
+
+ alloc_size = min_t(unsigned long, needed_sges, SG_MAX_SINGLE_ALLOC);
+ new_sg = sg_kmalloc(alloc_size, gfp_mask);
+ if (!new_sg)
+ return ERR_PTR(-ENOMEM);
+ sg_init_table(new_sg, alloc_size);
+ if (cur) {
+ __sg_chain(cur, new_sg);
+ table->orig_nents += alloc_size - 1;
+ } else {
+ table->sgl = new_sg;
+ table->orig_nents = alloc_size;
+ table->nents = 0;
+ }
+ return new_sg;
+}
+
/**
* __sg_alloc_table_from_pages - Allocate and initialize an sg table from
* an array of pages
@@ -374,29 +412,64 @@ EXPORT_SYMBOL(sg_alloc_table);
* @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)
+ * @prv: Last populated sge in sgt
+ * @left_pages: Left pages caller have to set after this call
* @gfp_mask: GFP allocation mask
*
- * Description:
- * Allocate 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. An user may provide an offset at a
- * start and a size of valid data in a buffer specified by the page array.
- * The returned sg table is released by sg_free_table.
+ * Description:
+ * If @prv is NULL, allocate and initialize an sg table from a list of pages,
+ * else reuse the scatterlist passed in at @prv.
+ * Contiguous ranges of the pages are squashed into a single scatterlist
+ * entry 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.
*
* Returns:
- * 0 on success, negative error on failure
+ * Last SGE in sgt on success, PTR_ERR on otherwise.
+ * The allocation in @sgt must be released by sg_free_table.
+ *
+ * Notes:
+ * If this function returns non-0 (eg failure), the caller must call
+ * sg_free_table() to cleanup any leftover allocations.
*/
-int __sg_alloc_table_from_pages(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 *__sg_alloc_table_from_pages(struct sg_table *sgt,
+ struct page **pages, unsigned int n_pages, unsigned int offset,
+ unsigned long size, unsigned int max_segment,
+ struct scatterlist *prv, unsigned int left_pages,
+ gfp_t gfp_mask)
{
- unsigned int chunks, cur_page, seg_len, i;
- int ret;
- struct scatterlist *s;
+ unsigned int chunks, cur_page, seg_len, i, prv_len = 0;
+ unsigned int added_nents = 0;
+ struct scatterlist *s = prv;

if (WARN_ON(!max_segment || offset_in_page(max_segment)))
- return -EINVAL;
+ return ERR_PTR(-EINVAL);
+ if (IS_ENABLED(CONFIG_ARCH_NO_SG_CHAIN) && prv)
+ return ERR_PTR(-EOPNOTSUPP);
+
+ if (prv) {
+ unsigned long paddr = (page_to_pfn(sg_page(prv)) * PAGE_SIZE +
+ prv->offset + prv->length) /
+ PAGE_SIZE;
+
+ if (WARN_ON(offset))
+ return ERR_PTR(-EINVAL);
+
+ /* Merge contiguous pages into the last SG */
+ prv_len = prv->length;
+ while (n_pages && page_to_pfn(pages[0]) == paddr) {
+ if (prv->length + PAGE_SIZE > max_segment)
+ break;
+ prv->length += PAGE_SIZE;
+ paddr++;
+ pages++;
+ n_pages--;
+ }
+ if (!n_pages) {
+ sg_mark_end(sg_next(prv));
+ return prv;
+ }
+ }

/* compute number of contiguous chunks */
chunks = 1;
@@ -410,13 +483,9 @@ int __sg_alloc_table_from_pages(struct sg_table *sgt, struct page **pages,
}
}

- ret = sg_alloc_table(sgt, chunks, gfp_mask);
- if (unlikely(ret))
- return ret;
-
/* merging chunks and putting them into the scatterlist */
cur_page = 0;
- for_each_sg(sgt->sgl, s, sgt->orig_nents, i) {
+ for (i = 0; i < chunks; i++) {
unsigned int j, chunk_size;

/* look for the end of the current chunk */
@@ -429,15 +498,28 @@ int __sg_alloc_table_from_pages(struct sg_table *sgt, struct page **pages,
break;
}

+ /* Pass how many chunks might be left */
+ s = get_next_sg(sgt, s, chunks - i + left_pages, gfp_mask);
+ if (IS_ERR(s)) {
+ /*
+ * Adjust entry length to be as before function was
+ * called.
+ */
+ if (prv)
+ prv->length = prv_len;
+ return s;
+ }
chunk_size = ((j - cur_page) << PAGE_SHIFT) - offset;
sg_set_page(s, pages[cur_page],
min_t(unsigned long, size, chunk_size), offset);
+ added_nents++;
size -= chunk_size;
offset = 0;
cur_page = j;
}
-
- return 0;
+ sgt->nents += added_nents;
+ sg_mark_end(s);
+ return s;
}
EXPORT_SYMBOL(__sg_alloc_table_from_pages);