linux-kernel - Re: [lkp-robot] [x86/cpu_entry_area] 10043e02db: kernel_BUG_at

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Date:   Thu, 28 Dec 2017 19:01:41 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     kernel test robot <xiaolong.ye@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>,
        LKML <linux-kernel@...r.kernel.org>, tipbuild@...or.com,
        LKP <lkp@...org>, Alexander Potapenko <glider@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: [lkp-robot] [x86/cpu_entry_area] 10043e02db:
 kernel_BUG_at_arch/x86/mm/physaddr.c

On 12/27/2017 09:12 PM, Dmitry Vyukov wrote:

>>
>> Not really a problem caused by the patch above, it's merily exposing a code
>> path which relies blindly on "enough memory available" assumptions.
>>
>> Throwing more memory at the VM makes the problem go away...
> 
> Hi Thomas,
> 
> We just need a check inside of early_alloc() to properly diagnose such
> situation, right?
> KASAN allocates 1/8 of physical memory, so the code has _some_ logic
> behind it. But I guess it runs into a corner case where 1/8 is somehow
> larger than 1 :)
> 

We may simply don't have 1/8 available. And we actually need slightly more
than 1/8. Kernel image is present in 2 locations (direct mapping/kernel mapping)
so at least ram/8 + kernel_size/8. The same for cpu_entry_area. Plus some memory
to build page tables.
And note that kernel_size could be hundreds of megabytes.