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Subject[PATCH rdma-next v1 1/4] lib/scatterlist: Refactor sg_alloc_table_from_pages
Date
From: Maor Gottlieb <maorg@nvidia.com>

Currently, sg_alloc_table_from_pages doesn't support dynamic chaining of
SG entries. Therefore it requires from user to allocate all the pages in
advance and hold them in a large buffer. Such a buffer consumes a lot of
temporary memory in HPC systems which do a very large memory registration.

The next patches introduce API for dynamically allocation from pages and
it requires us to do the following:
* Extract the code to alloc_from_pages_common.
* Change the build of the table to iterate on the chunks and not on the
SGEs. It will allow dynamic allocation of more SGEs.

Since sg_alloc_table_from_pages allocate exactly the number of chunks,
therefore chunks are equal to the number of SG entries.

Signed-off-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
---
lib/scatterlist.c | 75 ++++++++++++++++++++++++++++-------------------
1 file changed, 45 insertions(+), 30 deletions(-)

diff --git a/lib/scatterlist.c b/lib/scatterlist.c
index 5d63a8857f36..292e785d21ee 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -365,38 +365,18 @@ int sg_alloc_table(struct sg_table *table, unsigned int nents, gfp_t gfp_mask)
}
EXPORT_SYMBOL(sg_alloc_table);

-/**
- * __sg_alloc_table_from_pages - 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
- *
- * 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.
- *
- * Returns:
- * 0 on success, negative error on failure
- */
-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)
+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)
{
unsigned int chunks, cur_page, seg_len, i;
+ struct scatterlist *prv, *s = NULL;
int ret;
- struct scatterlist *s;

if (WARN_ON(!max_segment || offset_in_page(max_segment)))
- return -EINVAL;
+ return ERR_PTR(-EINVAL);

/* compute number of contiguous chunks */
chunks = 1;
@@ -412,11 +392,12 @@ 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;
+ return ERR_PTR(ret);

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

/* look for the end of the current chunk */
@@ -435,9 +416,43 @@ int __sg_alloc_table_from_pages(struct sg_table *sgt, struct page **pages,
size -= chunk_size;
offset = 0;
cur_page = j;
+ prv = s;
+ s = sg_next(s);
}
+ return prv;
+}

- return 0;
+/**
+ * __sg_alloc_table_from_pages - 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
+ *
+ * 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. A 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.
+ *
+ * Returns:
+ * 0 on success, negative error on failure
+ */
+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;
+
+ sg = alloc_from_pages_common(sgt, pages, n_pages, offset, size,
+ max_segment, gfp_mask);
+ return PTR_ERR_OR_ZERO(sg);
}
EXPORT_SYMBOL(__sg_alloc_table_from_pages);

--
2.26.2
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