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[220.245.128.230]) by smtp.gmail.com with ESMTPSA id f13sm13736786pfa.132.2019.02.16.22.23.36 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Sat, 16 Feb 2019 22:23:37 -0800 (PST) Date: Sun, 17 Feb 2019 17:23:33 +1100 From: Balbir Singh To: Segher Boessenkool Cc: Michael Ellerman , erhard_f@mailbox.org, jack@suse.cz, linuxppc-dev@ozlabs.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, aneesh.kumar@linux.vnet.ibm.com Subject: Re: [PATCH] powerpc/64s: Fix possible corruption on big endian due to pgd/pud_present() Message-ID: <20190217062333.GC31125@350D> References: <20190214062339.7139-1-mpe@ellerman.id.au> <20190216105511.GA31125@350D> <20190216142206.GE14180@gate.crashing.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190216142206.GE14180@gate.crashing.org> User-Agent: Mutt/1.9.4 (2018-02-28) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Sat, Feb 16, 2019 at 08:22:12AM -0600, Segher Boessenkool wrote: > Hi all, > > On Sat, Feb 16, 2019 at 09:55:11PM +1100, Balbir Singh wrote: > > On Thu, Feb 14, 2019 at 05:23:39PM +1100, Michael Ellerman wrote: > > > In v4.20 we changed our pgd/pud_present() to check for _PAGE_PRESENT > > > rather than just checking that the value is non-zero, e.g.: > > > > > > static inline int pgd_present(pgd_t pgd) > > > { > > > - return !pgd_none(pgd); > > > + return (pgd_raw(pgd) & cpu_to_be64(_PAGE_PRESENT)); > > > } > > > > > > Unfortunately this is broken on big endian, as the result of the > > > bitwise && is truncated to int, which is always zero because > > (Bitwise "&" of course). > > > Not sure why that should happen, why is the result an int? What > > causes the casting of pgd_t & be64 to be truncated to an int. > > Yes, it's not obvious as written... It's simply that the return type of > pgd_present is int. So it is truncated _after_ the bitwise and. > Thanks, I am surprised the compiler does not complain about the truncation of bits. I wonder if we are missing -Wconversion Balbir