linux-kernel - Re: [PATCH v2 1/1] mm: kfence: apply kmemleak_ignore

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Date:   Tue, 19 Jul 2022 16:22:12 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Marco Elver <elver@...gle.com>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>, yee.lee@...iatek.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        "open list:KFENCE" <kasan-dev@...glegroups.com>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-arm-kernel@...ts.infradead.org>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-mediatek@...ts.infradead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v2 1/1] mm: kfence: apply kmemleak_ignore_phys on early
 allocated pool

On 7/19/22 16:13, Andrew Morton wrote:
> On Mon, 18 Jul 2022 16:26:25 +0200 Marco Elver <elver@...gle.com> wrote:
> 
>> On Sat, 16 Jul 2022 at 20:43, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>> [...]
>>>> - This patch has been accused of crashing the kernel:
>>>>
>>>>         https://lkml.kernel.org/r/YsFeUHkrFTQ7T51Q@xsang-OptiPlex-9020
>>>>
>>>>   Do we think that report is bogus?
>>> I think all of this is highly architecture-specific...
>> The report can be reproduced on i386 with CONFIG_X86_PAE=y. But e.g.
>> mm/memblock.c:memblock_free() is also guilty of using __pa() on
>> previously memblock_alloc()'d addresses. Looking at the phys addr
>> before memblock_alloc() does virt_to_phys(), the result of __pa()
>> looks correct even on PAE, at least for the purpose of passing it on
>> to kmemleak(). So I don't know what that BUG_ON(slow_virt_to_phys() !=
>> phys_addr) is supposed to tell us here.
>>
> It's only been nine years, so I'm sure Dave can remember why he added
> it ;)
> 
> 		BUG_ON(slow_virt_to_phys((void *)x) != phys_addr);
> 
> in arch/x86/mm/physaddr.c:__phys_addr().

I think I intended it to double check that the linear map is *actually*
a linear map for 'x'.  Sure, we can use the "x - PAGE_OFFSET" shortcut,
but did it turn out to be actually accurate for the address it was handed?

I'd be curious what the page tables actually say for the address that's
causing problems.