[PATCH v2 18/21] xen: gntdev: fix sg_table nents vs. orig_nents misuse

[Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>]

The Documentation/DMA-API-HOWTO.txt states that dma_map_sg returns the
numer of the created entries in the DMA address space. However the
subsequent calls to dma_sync_sg_for_{device,cpu} and dma_unmap_sg must be
called with the original number of entries passed to dma_map_sg. The
sg_table->nents in turn holds the result of the dma_map_sg call as stated
in include/linux/scatterlist.h. Adapt the code to obey those rules.

Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
---
For more information, see '[PATCH v2 00/21] DRM: fix struct sg_table nents
vs. orig_nents misuse' thread: https://lkml.org/lkml/2020/5/4/373
---
 drivers/xen/gntdev-dmabuf.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/xen/gntdev-dmabuf.c b/drivers/xen/gntdev-dmabuf.c
index 75d3bb9..ed749fd 100644
--- a/drivers/xen/gntdev-dmabuf.c
+++ b/drivers/xen/gntdev-dmabuf.c
@@ -248,7 +248,7 @@ static void dmabuf_exp_ops_detach(struct dma_buf *dma_buf,
                if (sgt) {
                        if (gntdev_dmabuf_attach->dir != DMA_NONE)
                                dma_unmap_sg_attrs(attach->dev, sgt->sgl,
-                                                  sgt->nents,
+                                                  sgt->orig_nents,
                                                   gntdev_dmabuf_attach->dir,
                                                   DMA_ATTR_SKIP_CPU_SYNC);
                        sg_free_table(sgt);
@@ -288,8 +288,10 @@ static void dmabuf_exp_ops_detach(struct dma_buf *dma_buf,
        sgt = dmabuf_pages_to_sgt(gntdev_dmabuf->pages,
                                  gntdev_dmabuf->nr_pages);
        if (!IS_ERR(sgt)) {
-               if (!dma_map_sg_attrs(attach->dev, sgt->sgl, sgt->nents, dir,
-                                     DMA_ATTR_SKIP_CPU_SYNC)) {
+               sgt->nents = dma_map_sg_attrs(attach->dev, sgt->sgl,
+                                             sgt->orig_nents, dir,
+                                             DMA_ATTR_SKIP_CPU_SYNC);
+               if (!sgt->nents) {
                        sg_free_table(sgt);
                        kfree(sgt);
                        sgt = ERR_PTR(-ENOMEM);
@@ -625,7 +627,7 @@ static struct gntdev_dmabuf *dmabuf_imp_alloc_storage(int 
count)
 
        /* Now convert sgt to array of pages and check for page validity. */
        i = 0;
-       for_each_sg_page(sgt->sgl, &sg_iter, sgt->nents, 0) {
+       for_each_sg_page(sgt->sgl, &sg_iter, sgt->orig_nents, 0) {
                struct page *page = sg_page_iter_page(&sg_iter);
                /*
                 * Check if page is valid: this can happen if we are given
-- 
1.9.1