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

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Date:   Tue, 19 Jul 2022 16:13:56 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     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 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().


This kfence patch does seem to be desirable, but we can't proceed if
it's resulting in kernel crashes.