Re: [RFC PATCH bpf-next v2 4/4] error-injection: Support fault injection framework

From: Alexei Starovoitov
Date: Wed Dec 27 2017 - 22:50:11 EST

On 12/27/17 5:38 PM, Masami Hiramatsu wrote:
On Wed, 27 Dec 2017 14:49:46 -0800
Alexei Starovoitov <ast@xxxxxx> wrote:

On 12/27/17 12:09 AM, Masami Hiramatsu wrote:
On Tue, 26 Dec 2017 18:12:56 -0800
Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote:

On Tue, Dec 26, 2017 at 04:48:25PM +0900, Masami Hiramatsu wrote:
Support in-kernel fault-injection framework via debugfs.
This allows you to inject a conditional error to specified
function using debugfs interfaces.

Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Documentation/fault-injection/fault-injection.txt | 5 +
kernel/Makefile | 1
kernel/fail_function.c | 169 +++++++++++++++++++++
lib/Kconfig.debug | 10 +
4 files changed, 185 insertions(+)
create mode 100644 kernel/fail_function.c

diff --git a/Documentation/fault-injection/fault-injection.txt b/Documentation/fault-injection/fault-injection.txt
index 918972babcd8..6243a588dd71 100644
--- a/Documentation/fault-injection/fault-injection.txt
+++ b/Documentation/fault-injection/fault-injection.txt
@@ -30,6 +30,11 @@ o fail_mmc_request
injects MMC data errors on devices permitted by setting
debugfs entries under /sys/kernel/debug/mmc0/fail_mmc_request

+o fail_function
+ injects error return on specific functions by setting debugfs entries
+ under /sys/kernel/debug/fail_function. No boot option supported.

I like it.
Could you document it a bit better?

Yes, I will do in next series.

In particular retval is configurable, but without an example no one
will be able to figure out how to use it.

Ah, right. BTW, as I pointed in the covermail, should we store the
expected error value range into the injectable list? e.g.


And provide APIs to check/get it.

I'm afraid such check would be too costly.
Right now we have only two functions marked but I expect hundreds more
will be added in the near future as soon as developers realize the
potential of such error injection.
All of ALLOW_ERROR_INJECTION marks add 8 byte overhead each to .data.
Multiple by 1k and we have 8k of data spent on marks.
If we add max/min range marks that doubles it for very little use.
I think marking function only is enough.

Sorry, I don't think so.
Even if it takes 16 bytes more for each points, I don't think it is
any overhead for machines in these days. Even if so, we can provide
a kconfig to reduce it.
I mean, we are living in GB-order memory are, and it will be bigger
in the future. Why we have to worry about hundreds of 16bytes memory
pieces? It will take a few KB, and even if we mark thousands of
functions, it never reaches 1MB, in GB memory pool. :)

Of course, for many small-footprint embedded devices (like having
less than 128MB memory), this feature can be a overhead. But they
can cut off the table by kconfig.

I still disagree on wasting 16-byte * num_of_funcs of .data here.
The trade-off of usability vs memory just not worth it. Sorry.
Please focus on testing your changes instead.