Re: [PATCH][RFC] x86/intel_rdt: Do not display size for non-CAT resource

From: Reinette Chatre <reinette.chatre@intel.com>
To: Chen Yu <yu.c.chen@intel.com>, Fenghua Yu <fenghua.yu@intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
	Tony Luck <tony.luck@intel.com>
Subject: Re: [PATCH][RFC] x86/intel_rdt: Do not display size for non-CAT resource
Date: Tue, 4 Sep 2018 13:24:11 -0700	[thread overview]
Message-ID: <8cf4c10f-68fe-04a5-029c-9cb22db413ff@intel.com> (raw)
In-Reply-To: <20180904174614.26682-1-yu.c.chen@intel.com>

Hi Chen Yu,

On 9/4/2018 10:46 AM, Chen Yu wrote:
> On a platform with MB resource enabled, a divided-by-zero
> exception is triggered when accessing 'size':
> 
> [  151.193447] divide error: 0000 [#1] SMP PTI
> [  151.197743] CPU: 93 PID: 1929 Comm: cat Not tainted 4.19.0-rc2-debug-rdt+ #25
> [  151.205070] Hardware name: Dell Inc. PowerEdge R640/0CRT1G, BIOS 1.3.7 02/08/2018
> [  151.212783] RIP: 0010:rdtgroup_cbm_to_size+0x7e/0xa0
> [  151.237172] RSP: 0018:ffffb3454f90bd88 EFLAGS: 00010246
> [  151.242538] RAX: 00000000023c0000 RBX: 0000000000000000 RCX: 0000000000000003
> [  151.249878] RDX: 0000000000000000 RSI: 0000000000000004 RDI: 0000000000000003
> [  151.257213] RBP: ffff96ff0089e000 R08: 0000000000000000 R09: 0000000000aaaaaa
> [  151.264544] R10: ffffb3454f90bd8c R11: 00000000ffffffff R12: ffffffffb5028910
> [  151.271887] R13: ffffffffb5028910 R14: 0000000000000064 R15: ffff96ff0089e000
> [  151.279217] FS:  00007f95a623a500(0000) GS:ffff97170f9c0000(0000) knlGS:0000000000000000
> [  151.287532] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [  151.293432] CR2: 00007f95a6217000 CR3: 00000023f696c003 CR4: 00000000007606e0
> [  151.300766] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [  151.308094] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [  151.315426] PKRU: 55555554
> [  151.318212] Call Trace:
> [  151.320732]  rdtgroup_size_show+0x11a/0x1d0
> [  151.325039]  seq_read+0xd8/0x3b0
> [  151.328363]  __vfs_read+0x36/0x170
> [  151.331857]  vfs_read+0x89/0x130
> [  151.335179]  ksys_read+0x52/0xc0
> [  151.338500]  do_syscall_64+0x5b/0x180
> [  151.342261]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> This is because for MB resource, the r->cache.cbm_len is zero, thus
> calculating size in rdtgroup_cbm_to_size() will trigger the exception.
> 
> Fix this issue by not exposing 'size' for non-CAT resources.
> 
> Fixes: d9b48c86eb38 ("x86/intel_rdt: Display resource groups'
> allocations in bytes")
> Cc: Reinette Chatre <reinette.chatre@intel.com>
> Cc: Fenghua Yu <fenghua.yu@intel.com>
> Cc: Tony Luck <tony.luck@intel.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Chen Yu <yu.c.chen@intel.com>
> ---
>  arch/x86/kernel/cpu/intel_rdt_rdtgroup.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/intel_rdt_rdtgroup.c b/arch/x86/kernel/cpu/intel_rdt_rdtgroup.c
> index b799c00bef09..53fd07b2f61a 100644
> --- a/arch/x86/kernel/cpu/intel_rdt_rdtgroup.c
> +++ b/arch/x86/kernel/cpu/intel_rdt_rdtgroup.c
> @@ -1329,7 +1329,7 @@ static struct rftype res_common_files[] = {
>  		.mode		= 0444,
>  		.kf_ops		= &rdtgroup_kf_single_ops,
>  		.seq_show	= rdtgroup_size_show,
> -		.fflags		= RF_CTRL_BASE,
> +		.fflags		= RF_CTRL_INFO | RFTYPE_RES_CACHE,
>  	},
>  
>  };
> 

Thank you very much for catching this.

I think we need to change the fix a bit because from that I can tell the
above would cause the "size" file to be relocated to the system wide
"info" directory while we would like to have this file remain associated
with the resource group - but just not apply to a MB resource.

A similar fix may also be needed for the resource group's "mode" file
that was also recently introduced.

I am taking a closer look now.

Reinette