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Thu, 18 Jun 2020 16:22:11 +0200 (CEST) Subject: Re: [PATCH 3/3] powerpc/8xx: Provide ptep_get() with 16k pages To: Michael Ellerman , Peter Zijlstra Cc: Benjamin Herrenschmidt , Paul Mackerras , Will Deacon , Andrew Morton , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org References: <341688399c1b102756046d19ea6ce39db1ae4742.1592225558.git.christophe.leroy@csgroup.eu> <20200615132244.GR2531@hirez.programming.kicks-ass.net> <87wo45db8d.fsf@mpe.ellerman.id.au> <20200617143826.GJ2531@hirez.programming.kicks-ass.net> <87pn9xchql.fsf@mpe.ellerman.id.au> From: Christophe Leroy Message-ID: Date: Thu, 18 Jun 2020 16:21:16 +0200 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0 MIME-Version: 1.0 In-Reply-To: <87pn9xchql.fsf@mpe.ellerman.id.au> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: fr X-Rspamd-Queue-Id: 6812C180295BC X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam01 Content-Transfer-Encoding: quoted-printable 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: Le 18/06/2020 =C3=A0 02:58, Michael Ellerman a =C3=A9crit=C2=A0: > Peter Zijlstra writes: >> On Thu, Jun 18, 2020 at 12:21:22AM +1000, Michael Ellerman wrote: >>> Peter Zijlstra writes: >>>> On Mon, Jun 15, 2020 at 12:57:59PM +0000, Christophe Leroy wrote: >> >>>>> +#if defined(CONFIG_PPC_8xx) && defined(CONFIG_PPC_16K_PAGES) >>>>> +#define __HAVE_ARCH_PTEP_GET >>>>> +static inline pte_t ptep_get(pte_t *ptep) >>>>> +{ >>>>> + pte_t pte =3D {READ_ONCE(ptep->pte), 0, 0, 0}; >>>>> + >>>>> + return pte; >>>>> +} >>>>> +#endif >>>> >>>> Would it make sense to have a comment with this magic? The casual re= ader >>>> might wonder WTH just happened when he stumbles on this :-) >>> >>> I tried writing a helpful comment but it's too late for my brain to f= orm >>> sensible sentences. >>> >>> Christophe can you send a follow-up with a comment explaining it? In >>> particular the zero entries stand out, it's kind of subtle that those >>> entries are only populated with the right value when we write to the >>> page table. >> >> static inline pte_t ptep_get(pte_t *ptep) >> { >> unsigned long val =3D READ_ONCE(ptep->pte); >> /* 16K pages have 4 identical value 4K entries */ >> pte_t pte =3D {val, val, val, val); >> return pte; >> } >> >> Maybe something like that? >=20 > I think val wants to be pte_basic_t, but otherwise yeah I like that muc= h > better. >=20 I sent a patch for that. I'll also send one to fix mm/debug_vm_pgtable.c which also uses=20 READ_ONCE() to access page table entries. Christophe