[PATCH-RESEND] cxl: Disable prefault_mode in Radix mode

* [PATCH-RESEND] cxl: Disable prefault_mode in Radix mode
@ 2018-05-18  9:42 Vaibhav Jain
  2018-05-18 12:43 ` Frederic Barrat
                   ` (3 more replies)
  0 siblings, 4 replies; 6+ messages in thread
From: Vaibhav Jain @ 2018-05-18  9:42 UTC (permalink / raw)
  To: linuxppc-dev, Frederic Barrat
  Cc: Vaibhav Jain, Andrew Donnellan, Christophe Lombard,
	Philippe Bergheaud, Alastair D'Silva, Vaibhav Jain, stable

From: Vaibhav Jain <vaibhav@linux.ibm.com>

Currently we see a kernel-oops reported on Power-9 while attaching a
context to an AFU, with radix-mode and sysfs attr 'prefault_mode' set
to anything other than 'none'. The backtrace of the oops is of this
form:

Unable to handle kernel paging request for data at address 0x00000080
Faulting instruction address: 0xc00800000bcf3b20
cpu 0x1: Vector: 300 (Data Access) at [c00000037f003800]
    pc: c00800000bcf3b20: cxl_load_segment+0x178/0x290 [cxl]
    lr: c00800000bcf39f0: cxl_load_segment+0x48/0x290 [cxl]
    sp: c00000037f003a80
   msr: 9000000000009033
   dar: 80
 dsisr: 40000000
  current = 0xc00000037f280000
  paca    = 0xc0000003ffffe600   softe: 3        irq_happened: 0x01
    pid   = 3529, comm = afp_no_int
<snip>
[c00000037f003af0] c00800000bcf4424 cxl_prefault+0xfc/0x248 [cxl]
[c00000037f003b50] c00800000bcf8a40 process_element_entry_psl9+0xd8/0x1a0 [cxl]
[c00000037f003b90] c00800000bcf944c cxl_attach_dedicated_process_psl9+0x44/0x130 [cxl]
[c00000037f003bd0] c00800000bcf5448 native_attach_process+0xc0/0x130 [cxl]
[c00000037f003c50] c00800000bcf16cc afu_ioctl+0x3f4/0x5e0 [cxl]
[c00000037f003d00] c00000000039d98c do_vfs_ioctl+0xdc/0x890
[c00000037f003da0] c00000000039e1a8 ksys_ioctl+0x68/0xf0
[c00000037f003df0] c00000000039e270 sys_ioctl+0x40/0xa0
[c00000037f003e30] c00000000000b320 system_call+0x58/0x6c
--- Exception: c01 (System Call) at 0000000010053bb0

The issue is caused as on Power-8 the AFU attr 'prefault_mode' was
used to improve initial storage fault performance by prefaulting
process segments. However on Power-9 with radix mode we don't have
Storage-Segments that we can prefault. Also prefaulting process Pages
will be too costly and fine-grained.

Hence, since the prefaulting mechanism doesn't makes sense of
radix-mode, this patch updates prefault_mode_store() to not allow any
other value apart from CXL_PREFAULT_NONE when radix mode is enabled.

Cc: <stable@vger.kernel.org>
Fixes: f24be42aab37 ("cxl: Add psl9 specific code")
Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
---
Change-log:

Resend  ->  Updated the commit description to add more info on the
	    issue seen [Andrew]
---
 Documentation/ABI/testing/sysfs-class-cxl |  4 +++-
 drivers/misc/cxl/sysfs.c                  | 16 ++++++++++++----
 2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-class-cxl b/Documentation/ABI/testing/sysfs-class-cxl
index 640f65e79ef1..267920a1874b 100644
--- a/Documentation/ABI/testing/sysfs-class-cxl
+++ b/Documentation/ABI/testing/sysfs-class-cxl
@@ -69,7 +69,9 @@ Date:           September 2014
 Contact:        linuxppc-dev@lists.ozlabs.org
 Description:    read/write
                 Set the mode for prefaulting in segments into the segment table
-                when performing the START_WORK ioctl. Possible values:
+                when performing the START_WORK ioctl. Only applicable when
+                running under hashed page table mmu.
+                Possible values:
                         none: No prefaulting (default)
                         work_element_descriptor: Treat the work element
                                  descriptor as an effective address and
diff --git a/drivers/misc/cxl/sysfs.c b/drivers/misc/cxl/sysfs.c
index 4b5a4c5d3c01..629e2e156412 100644
--- a/drivers/misc/cxl/sysfs.c
+++ b/drivers/misc/cxl/sysfs.c
@@ -353,12 +353,20 @@ static ssize_t prefault_mode_store(struct device *device,
 	struct cxl_afu *afu = to_cxl_afu(device);
 	enum prefault_modes mode = -1;
 
-	if (!strncmp(buf, "work_element_descriptor", 23))
-		mode = CXL_PREFAULT_WED;
-	if (!strncmp(buf, "all", 3))
-		mode = CXL_PREFAULT_ALL;
 	if (!strncmp(buf, "none", 4))
 		mode = CXL_PREFAULT_NONE;
+	else {
+		if (!radix_enabled()) {
+
+			/* only allowed when not in radix mode */
+			if (!strncmp(buf, "work_element_descriptor", 23))
+				mode = CXL_PREFAULT_WED;
+			if (!strncmp(buf, "all", 3))
+				mode = CXL_PREFAULT_ALL;
+		} else {
+			dev_err(device, "Cannot prefault with radix enabled\n");
+		}
+	}
 
 	if (mode == -1)
 		return -EINVAL;
-- 
2.17.0

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