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

[Frederic Barrat]

Le 18/05/2018 à 11:42, Vaibhav Jain a écrit :
> From: Vaibhav Jain <vaibhav at 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 at vger.kernel.org>
> Fixes: f24be42aab37 ("cxl: Add psl9 specific code")
> Signed-off-by: Vaibhav Jain <vaibhav at linux.ibm.com>
> ---

Thanks!
Acked-by: Frederic Barrat <fbarrat at linux.vnet.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 at 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;
>