linux-kernel - Re: [PATCH 1/2] mm/zsmalloc.c: check encoded object value overflow for PAE

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Date:   Thu, 25 Oct 2018 13:00:06 +0100
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Rafael David Tinoco <rafael.tinoco@...aro.org>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mm@...ck.org, Mark Brown <broonie@...nel.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Nitin Gupta <ngupta@...are.org>,
        Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH 1/2] mm/zsmalloc.c: check encoded object value overflow
 for PAE

On Wed, Oct 24, 2018 at 10:27:44PM -0300, Rafael David Tinoco wrote:
> On 32-bit systems, zsmalloc uses HIGHMEM and, when PAE is enabled, the
> physical frame number might be so big that zsmalloc obj encoding (to
> location) will break IF architecture does not re-define
> MAX_PHYSMEM_BITS, causing:

I think there's a deeper problem here - a misunderstanding of what
MAX_PHYSMEM_BITS is.

MAX_PHYSMEM_BITS is a definition for sparsemem, and is only visible
when sparsemem is enabled.  When sparsemem is disabled, asm/sparsemem.h
is not included (and should not be included) which means there is no
MAX_PHYSMEM_BITS definition.

I don't think zsmalloc.c should be (ab)using MAX_PHYSMEM_BITS, and
your description above makes it sound like you expect it to always be
defined.

If we want to have a definition for this, we shouldn't be playing
fragile games like:

#ifndef MAX_POSSIBLE_PHYSMEM_BITS
#ifdef MAX_PHYSMEM_BITS
#define MAX_POSSIBLE_PHYSMEM_BITS MAX_PHYSMEM_BITS
#else
/*
 * If this definition of MAX_PHYSMEM_BITS is used, OBJ_INDEX_BITS will just
 * be PAGE_SHIFT
 */
#define MAX_POSSIBLE_PHYSMEM_BITS BITS_PER_LONG
#endif
#endif

but instead insist that MAX_PHYSMEM_BITS is defined _everywhere_.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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