Re: missing clear bdr in check_ram_in_range_map()?

[Christoph Hellwig <hch@lst.de>]

On Mon, Dec 09, 2024 at 06:50:06PM +0800, Baochen Qiang wrote:
> Hi,
> 
> while checking check_ram_in_range_map() I am confused by the condition set/check on bdr.
> If I am reading the code correctly, if bdr is set once, it would never get cleared, hence
> that function will always returns 0.
> 
> should we clear bdr before each new iteration?

I think so.  Even better refactor the code so that the non-NULL bdr
doesn't leak out:

diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
index 5b4e6d3bf7bc..181e244f410a 100644
--- a/kernel/dma/direct.c
+++ b/kernel/dma/direct.c
@@ -584,6 +584,22 @@ int dma_direct_supported(struct device *dev, u64 mask)
 	return mask >= phys_to_dma_unencrypted(dev, min_mask);
 }
 
+static const struct bus_dma_region *dma_find_range(struct device *dev,
+		unsigned long start_pfn)
+{
+	const struct bus_dma_region *m;
+
+	for (m = dev->dma_range_map; PFN_DOWN(m->size); m++) {
+		unsigned long cpu_start_pfn = PFN_DOWN(m->cpu_start);
+
+		if (start_pfn >= cpu_start_pfn &&
+		    start_pfn - cpu_start_pfn < PFN_DOWN(m->size))
+			return m;
+	}
+
+	return NULL;
+}
+
 /*
  * To check whether all ram resource ranges are covered by dma range map
  * Returns 0 when further check is needed
@@ -593,23 +609,14 @@ static int check_ram_in_range_map(unsigned long start_pfn,
 				  unsigned long nr_pages, void *data)
 {
 	unsigned long end_pfn = start_pfn + nr_pages;
-	const struct bus_dma_region *bdr = NULL;
-	const struct bus_dma_region *m;
 	struct device *dev = data;
 
 	while (start_pfn < end_pfn) {
-		for (m = dev->dma_range_map; PFN_DOWN(m->size); m++) {
-			unsigned long cpu_start_pfn = PFN_DOWN(m->cpu_start);
+		const struct bus_dma_region *bdr;
 
-			if (start_pfn >= cpu_start_pfn &&
-			    start_pfn - cpu_start_pfn < PFN_DOWN(m->size)) {
-				bdr = m;
-				break;
-			}
-		}
+		bdr = dma_find_range(dev, start_pfn);
 		if (!bdr)
 			return 1;
-
 		start_pfn = PFN_DOWN(bdr->cpu_start) + PFN_DOWN(bdr->size);
 	}