__sg_alloc_table_from

/**
 * __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)
{
    unsigned int chunks, cur_page, seg_len, i;
    int ret;
    struct scatterlist *s;
 
    if (WARN_ON(!max_segment || offset_in_page(max_segment)))
        return -EINVAL;
 
    /* compute number of contiguous chunks */
    chunks = 1;
    seg_len = 0;
    for (i = 1; i < n_pages; i++) {
        seg_len += PAGE_SIZE;
        if (seg_len >= max_segment ||
            page_to_pfn(pages[i]) != page_to_pfn(pages[i - 1]) + 1) {
            chunks++;
            seg_len = 0;
        }
    }
 
    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) {
        unsigned int j, chunk_size;
 
        /* look for the end of the current chunk */
        seg_len = 0;
        for (j = cur_page + 1; j < n_pages; j++) {
            seg_len += PAGE_SIZE;
            if (seg_len >= max_segment ||
                page_to_pfn(pages[j]) !=
                page_to_pfn(pages[j - 1]) + 1)
                break;
        }
 
        chunk_size = ((j - cur_page) << PAGE_SHIFT) - offset;
        sg_set_page(s, pages[cur_page],
                min_t(unsigned long, size, chunk_size), offset);
        size -= chunk_size;
        offset = 0;
        cur_page = j;
    }
 
    return 0;
}
函数源码
