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 09:37:59 -0300
From:   Rafael David Tinoco <rafael.tinoco@...aro.org>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Rafael David Tinoco <rafael.tinoco@...aro.org>,
        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

> 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.

Missed that part :\, tks.

> 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_.

Is it okay to propose using only MAX_PHYSMEM_BITS for zsmalloc (like
it was before commit 02390b87) instead, and make sure *at least* ARM
32/64 and x86/x64, for now, have it defined outside sparsemem headers
as well ? This way I can WARN_ONCE(), instead of BUG(), when specific
arch does not define it - enforcing behavior - showing BITS_PER_LONG
is being used instead of MAX_PHYSMEM_BITS (warning, at least once, for
the possibility of an overflow, like the issue showed in here).