guest-pin-stream: skip non-readable VMAs in the high-VA scan

A CUDA device buffer is a PROT_NONE reservation (often hundreds of MB).
Its pages are never present in the pagemap, so scanning it yields no
pairs - but it consumed the VMA_MAX_PAGES budget, and the scan never
reached the client's present staging buffers that sit ABOVE the device
reservation. Observed: a client whose device range started below its
staging buffer got no high-VA pins at all; its H2D copies failed rc=-2
forever and the client read back stale data (soak run: 9/9 green then
streams2 bad=32737). Skipping '---' VMAs restores the pins (9 fresh
high-VA injections in the first run; streams2 bad=0).

1 files changed, 12 insertions, 0 deletions

diff --git a/tools/guest-diag/guest_pin_stream.py b/tools/guest-diag/guest_pin_stream.py
index bb253a2..aa21655 100644
--- a/tools/guest-diag/guest_pin_stream.py
+++ b/tools/guest-diag/guest_pin_stream.py
@@ -1544,6 +1544,18 @@ def scan_vmas(fd, pid, out):
             parts = line.split()
             if len(parts) < 5:
                 continue
+            # Skip mappings with no read permission: a CUDA device buffer
+            # is a PROT_NONE reservation (e.g. `---p 00000000 00:00 0`),
+            # potentially hundreds of MB. Its pages are never present in
+            # the pagemap, so scanning it produces no pairs - but it
+            # consumed the VMA_MAX_PAGES budget and the scan never reached
+            # the client's present staging buffers that sit ABOVE the
+            # device reservation (observed: a client whose device range
+            # started below its staging buffer got no pins at all and its
+            # H2D copies failed with rc=-2 forever).
+            perms = parts[1] if len(parts) >= 2 else ''
+            if 'r' not in perms:
+                continue
             # Anonymous mappings (no path field) are the client's UVM sysmem
             # buffers when the driver maps them without a /dev/zero backing:
             # the second client's cudaMalloc'd data buffers show up exactly