linux-kernel - Re: [PATCH RFC 00/10] KFENCE: A low-overhead sampling-based memory safety error detector

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Date:   Tue, 8 Sep 2020 17:36:44 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Marco Elver <elver@...gle.com>, Dave Hansen <dave.hansen@...el.com>
Cc:     glider@...gle.com, akpm@...ux-foundation.org,
        catalin.marinas@....com, cl@...ux.com, rientjes@...gle.com,
        iamjoonsoo.kim@....com, mark.rutland@....com, penberg@...nel.org,
        hpa@...or.com, paulmck@...nel.org, andreyknvl@...gle.com,
        aryabinin@...tuozzo.com, luto@...nel.org, bp@...en8.de,
        dave.hansen@...ux.intel.com, dvyukov@...gle.com,
        edumazet@...gle.com, gregkh@...uxfoundation.org, mingo@...hat.com,
        jannh@...gle.com, corbet@....net, keescook@...omium.org,
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        will@...nel.org, x86@...nel.org, linux-doc@...r.kernel.org,
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        linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org
Subject: Re: [PATCH RFC 00/10] KFENCE: A low-overhead sampling-based memory
 safety error detector

On 9/8/20 5:31 PM, Marco Elver wrote:
>> 
>> How much memory overhead does this end up having?  I know it depends on
>> the object size and so forth.  But, could you give some real-world
>> examples of memory consumption?  Also, what's the worst case?  Say I
>> have a ton of worst-case-sized (32b) slab objects.  Will I notice?
> 
> KFENCE objects are limited (default 255). If we exhaust KFENCE's memory
> pool, no more KFENCE allocations will occur.
> Documentation/dev-tools/kfence.rst gives a formula to calculate the
> KFENCE pool size:
> 
> 	The total memory dedicated to the KFENCE memory pool can be computed as::
> 
> 	    ( #objects + 1 ) * 2 * PAGE_SIZE
> 
> 	Using the default config, and assuming a page size of 4 KiB, results in
> 	dedicating 2 MiB to the KFENCE memory pool.
> 
> Does that clarify this point? Or anything else that could help clarify
> this?

Hmm did you observe that with this limit, a long-running system would eventually
converge to KFENCE memory pool being filled with long-aged objects, so there
would be no space to sample new ones?

> Thanks,
> -- Marco
>