linux-kernel - Re: [RFC] ARC: mm: Restrict definition of pfn_valid() macro for CONFIG

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Date:   Wed, 30 Nov 2016 08:55:02 -0800
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Yuriy Kolerov <Yuriy.Kolerov@...opsys.com>,
        Michal Hocko <mhocko@...nel.org>
CC:     "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "Alexey.Brodkin@...opsys.com" <Alexey.Brodkin@...opsys.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] ARC: mm: Restrict definition of pfn_valid() macro for
 CONFIG_FLATMEM

On 11/30/2016 06:21 AM, Yuriy Kolerov wrote:
>> On Tue 29-11-16 18:29:06, Yuriy Kolerov wrote:
>>> > > Despite the fact that subtraction of unsigned integers is a defined
>>> > > behaviour however such operations can lead to unexpected results. Thus
>>> > > it is better to check both left and right boundaries to avoid
>>> > > potential bugs as it done in the generic page.h.
>> > 
>> > Why and which code would use an out of range pfn? Why other arches do
>> > not need to care?
> Actually some arches do care about checking of both left and right boundaries (e.g. avr32, sparc, etc). The problem is that a value of pfn may be calculated incorrectly in some places of the kernel. E.g. not long ago I sent a patch which fixes truncation of the most significant byte in pfn/pte in some cases (in the kernel with PAE40, however it is not a FLATMEM case). So such situations can happens in the most unexpected places.
> 

So the point is - is this a preventive fix (desired thing) or it being there would
have helped find the PAE40 bug earlier / easier. Woudl it have prevented the
kernel crash. If so then this is a nobrainer fix.

BTW did you try to gauge the code gen impact - this function gets pulled all over
the place in mm code. So build kernel with and w/o change and do a
scripts/bloat-o-meter

-Vineet