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Date:   Fri, 23 Sep 2022 10:34:28 +0300
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Subject: Re: [PATCH v2] mm: Make failslab writable again
To:     Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc:     Jonathan Corbet <corbet@lwn.net>, Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Roman Gushchin <roman.gushchin@linux.dev>, kernel@openvz.org,
        Kees Cook <keescook@chromium.org>,
        Roman Gushchin <guro@fb.com>, Jann Horn <jannh@google.com>,
        Vijayanand Jitta <vjitta@codeaurora.org>,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
References: <20220920121111.1792905-1-alexander.atanasov@virtuozzo.com>
 <Yyr1xONdw8dBgsKr@hyeyoo>
From:   Alexander Atanasov <alexander.atanasov@virtuozzo.com>
In-Reply-To: <Yyr1xONdw8dBgsKr@hyeyoo>
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Hello,

On 21.09.22 14:30, Hyeonggon Yoo wrote:
> On Tue, Sep 20, 2022 at 03:11:11PM +0300, Alexander Atanasov wrote:
>> In (060807f841ac mm, slub: make remaining slub_debug related attributes
>> read-only) failslab was made read-only.
>> I think it became a collateral victim to the two other options for which
>> the reasons are perfectly valid.
>> Here is why:
>>   - sanity_checks and trace are slab internal debug options,
>>     failslab is used for fault injection.
>>   - for fault injections, which by presumption are random, it
>>     does not matter if it is not set atomically. And you need to
>>     set atleast one more option to trigger fault injection.
>>   - in a testing scenario you may need to change it at runtime
>>     example: module loading - you test all allocations limited
>>     by the space option. Then you move to test only your module's
>>     own slabs.
>>   - when set by command line flags it effectively disables all
>>     cache merges.
> 
> Maybe we can make failslab= boot parameter to consider cache filtering?
> 
> With that, just pass something like this:
> 	failslab=X,X,X,X,cache_filter slub_debug=A,<cache-name>> 

> Users should pass slub_debug=A,<cache-name> anyway to prevent cache merging.
It will be good to have this in case you want to test cache that is used 
early. But why push something to command line option only when it can be 
changed at runtime?

-- 
Regards,
Alexander Atanasov