Re: [PATCH v2] selftests/mm: add simple VM_PFNMAP tests based on mmap'ing /dev/mem

[David Hildenbrand <david@redhat.com>]

On 28.05.25 12:53, Ryan Roberts wrote:
> On 28/05/2025 11:48, David Hildenbrand wrote:
>> On 28.05.25 12:44, David Hildenbrand wrote:
>>> On 28.05.25 12:34, Ryan Roberts wrote:
>>>> Hi David,
>>>>
>>>>
>>>> On 09/05/2025 16:30, David Hildenbrand wrote:
>>>>> Let's test some basic functionality using /dev/mem. These tests will
>>>>> implicitly cover some PAT (Page Attribute Handling) handling on x86.
>>>>>
>>>>> These tests will only run when /dev/mem access to the first two pages
>>>>> in physical address space is possible and allowed; otherwise, the tests
>>>>> are skipped.
>>>>
>>>> We are seeing really horrible RAS errors with this test when run on arm64 tx2
>>>> machine. Based solely on reviewing the code, I think the problem is that tx2
>>>> doesn't have anything at phys address 0, so test_read_access() is trying to put
>>>> trasactions out to a bad address on the bus.
>>>>
>>>> tx2 /proc/iomem:
>>>>
>>>> $ sudo cat /proc/iomem
>>>> 30000000-37ffffff : PCI ECAM
>>>> 38000000-3fffffff : PCI ECAM
>>>> 40000000-5fffffff : PCI Bus 0000:00
>>>> ...
>>>>
>>>> Whereas my x86 box has some reserved memory:
>>>>
>>>> $ sudo cat /proc/iomem
>>>> 00000000-00000fff : Reserved
>>>> 00001000-0003dfff : System RAM
>>>> ...
>>>>
>>>
>>> A quick fix would be to make this test specific to x86 (the only one I
>>> tested on). We should always have the lower two pages IIRC (BIOS stuff etc).
> 
> I'm not sure how far along this patch is? I'm guessing mm-stable? Perhaps you
> can do the quick fix, then I'd be happy to make this more robust for arm64 later?

Can you give the following a quick test on that machine? Then, I can send it as a
proper patch later.


 From 40fea063f2fcf1474fb47cb9aebdb04fd825032b Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@redhat.com>
Date: Wed, 28 May 2025 14:35:23 +0200
Subject: [PATCH] selftests/mm: two fixes for the pfnmap test

When unregistering the signal handler, we have to pass SIG_DFL, and
blindly reading from PFN 0 and PFN 1 seems to be problematic on !x86
systems. In particularly, on arm64 tx2 machines where noting resides
at these physical memory locations, we can generate RAS errors.

Let's fix it by scanning /proc/iomem for actual "System RAM".

Reported-by: Ryan Roberts <ryan.roberts@arm.com>
Closes: https://lore.kernel.org/all/232960c2-81db-47ca-a337-38c4bce5f997@arm.com/T/#u
Fixes: 2616b370323a ("selftests/mm: add simple VM_PFNMAP tests based on mmap'ing /dev/mem")
Signed-off-by: David Hildenbrand <david@redhat.com>
---
  tools/testing/selftests/mm/pfnmap.c | 61 +++++++++++++++++++++++++++--
  1 file changed, 57 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/mm/pfnmap.c b/tools/testing/selftests/mm/pfnmap.c
index 8a9d19b6020c7..4943927a7d1ea 100644
--- a/tools/testing/selftests/mm/pfnmap.c
+++ b/tools/testing/selftests/mm/pfnmap.c
@@ -12,6 +12,8 @@
  #include <stdint.h>
  #include <unistd.h>
  #include <errno.h>
+#include <stdio.h>
+#include <ctype.h>
  #include <fcntl.h>
  #include <signal.h>
  #include <setjmp.h>
@@ -43,14 +45,62 @@ static int test_read_access(char *addr, size_t size, size_t pagesize)
  			/* Force a read that the compiler cannot optimize out. */
  			*((volatile char *)(addr + offs));
  	}
-	if (signal(SIGSEGV, signal_handler) == SIG_ERR)
+	if (signal(SIGSEGV, SIG_DFL) == SIG_ERR)
  		return -EINVAL;
  
  	return ret;
  }
  
+static int find_ram_target(off_t *phys_addr,
+		unsigned long pagesize)
+{
+	unsigned long long start, end;
+	char line[80], *end_ptr;
+	FILE *file;
+
+	/* Search /proc/iomem for the first suitable "System RAM" range. */
+	file = fopen("/proc/iomem", "r");
+	if (!file)
+		return -errno;
+
+	while (fgets(line, sizeof(line), file)) {
+		/* Ignore any child nodes. */
+		if (!isalnum(line[0]))
+			continue;
+
+		if (!strstr(line, "System RAM\n"))
+			continue;
+
+		start = strtoull(line, &end_ptr, 16);
+		/* Skip over the "-" */
+		end_ptr++;
+		/* Make end "exclusive". */
+		end = strtoull(end_ptr, NULL, 16) + 1;
+
+		/* Actual addresses are not exported */
+		if (!start && !end)
+			break;
+
+		/* We need full pages. */
+		start = (start + pagesize - 1) & ~(pagesize - 1);
+		end &= ~(pagesize - 1);
+
+		if (start != (off_t)start)
+			break;
+
+		/* We need two pages. */
+		if (end > start + 2 * pagesize) {
+			fclose(file);
+			*phys_addr = start;
+			return 0;
+		}
+	}
+	return -ENOENT;
+}
+
  FIXTURE(pfnmap)
  {
+	off_t phys_addr;
  	size_t pagesize;
  	int dev_mem_fd;
  	char *addr1;
@@ -63,14 +113,17 @@ FIXTURE_SETUP(pfnmap)
  {
  	self->pagesize = getpagesize();
  
+	/* We'll require two physical pages throughout our tests ... */
+	if (find_ram_target(&self->phys_addr, self->pagesize))
+		SKIP(return, "Cannot find ram target in '/dev/iomem'\n");
+
  	self->dev_mem_fd = open("/dev/mem", O_RDONLY);
  	if (self->dev_mem_fd < 0)
  		SKIP(return, "Cannot open '/dev/mem'\n");
  
-	/* We'll require the first two pages throughout our tests ... */
  	self->size1 = self->pagesize * 2;
  	self->addr1 = mmap(NULL, self->size1, PROT_READ, MAP_SHARED,
-			   self->dev_mem_fd, 0);
+			   self->dev_mem_fd, self->phys_addr);
  	if (self->addr1 == MAP_FAILED)
  		SKIP(return, "Cannot mmap '/dev/mem'\n");
  
@@ -129,7 +182,7 @@ TEST_F(pfnmap, munmap_split)
  	 */
  	self->size2 = self->pagesize;
  	self->addr2 = mmap(NULL, self->pagesize, PROT_READ, MAP_SHARED,
-			   self->dev_mem_fd, 0);
+			   self->dev_mem_fd, self->phys_addr);
  	ASSERT_NE(self->addr2, MAP_FAILED);
  }
  
-- 
2.49.0


-- 
Cheers,

David / dhildenb