[PATCH v2 2/2] Xen/gnttab: improve error handling in gnttab_dma_{alloc,free}_pages()

[Juergen Gross]

From: Jan Beulich <jbeulich@xxxxxxxx>

Both decrease-reservation and populate-physmap can succeed partially. For
the former this may be pretty unlikely, but clearly the latter can hit
both out-of-memory conditions in the hypervisor or denial because of the
allocation exceeding the domain's allowance (there's no interaction with
the balloon driver here either).

In gnttab_dma_free_pages() we simply can't give back to the system what
hasn't been re-filled with backing memory. For gnttab_dma_alloc_pages()
both parts of the overall region need dealing with differently.

While no present caller of gnttab_dma_free_pages() checks its return
value, also don't use -EFAULT there: It really is an out-of-memory
condition. (In gnttab_dma_alloc_pages() -EFAULT doesn't look quite right
either, but I couldn't think of a clearly better error code there.)

Fixes: 9bdc7304f536 ("xen/grant-table: Allow allocating buffers suitable for DMA")
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 drivers/xen/grant-table.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
index 56a497987942..2a520080a570 100644
--- a/drivers/xen/grant-table.c
+++ b/drivers/xen/grant-table.c
@@ -1096,6 +1096,24 @@ int gnttab_dma_alloc_pages(struct gnttab_dma_alloc_args *args)
 	ret = xenmem_reservation_decrease(args->nr_pages, args->frames);
 	if (ret != args->nr_pages) {
 		pr_debug("Failed to decrease reservation for DMA buffer\n");
+		if (ret >= 0) {
+			/* Free the part where decrease didn't work. */
+			size_t done = ret << PAGE_SHIFT;
+
+			if (args->coherent)
+				dma_free_coherent(args->dev, size - done,
+						  args->vaddr + done,
+						  args->dev_bus_addr + done);
+			else
+				dma_free_wc(args->dev, size - done,
+					    args->vaddr + done,
+					    args->dev_bus_addr + done);
+
+			if (!ret) /* Nothing else to do? */
+				return -EFAULT;
+
+			args->nr_pages = ret;
+		}
 		ret = -EFAULT;
 		goto fail;
 	}
@@ -1132,7 +1150,12 @@ int gnttab_dma_free_pages(struct gnttab_dma_alloc_args *args)
 	ret = xenmem_reservation_increase(args->nr_pages, args->frames);
 	if (ret != args->nr_pages) {
 		pr_debug("Failed to increase reservation for DMA buffer\n");
-		ret = -EFAULT;
+		/* Can only free what has been re-filled. */
+		if (!ret)
+			return -ENOMEM;
+		if (ret > 0)
+			args->nr_pages = ret;
+		ret = -ENOMEM;
 	} else {
 		ret = 0;
 	}
-- 
2.56.0