From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt0-f199.google.com (mail-qt0-f199.google.com [209.85.216.199]) by kanga.kvack.org (Postfix) with ESMTP id AB7EC6B0279 for ; Fri, 16 Jun 2017 09:36:05 -0400 (EDT) Received: by mail-qt0-f199.google.com with SMTP id u51so35450795qte.15 for ; Fri, 16 Jun 2017 06:36:05 -0700 (PDT) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id i66si2012711qkd.164.2017.06.16.06.36.04 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 16 Jun 2017 06:36:04 -0700 (PDT) Date: Fri, 16 Jun 2017 15:36:00 +0200 From: Andrea Arcangeli Subject: Re: [PATCHv2 1/3] x86/mm: Provide pmdp_establish() helper Message-ID: <20170616133600.GE11676@redhat.com> References: <20170615145224.66200-1-kirill.shutemov@linux.intel.com> <20170615145224.66200-2-kirill.shutemov@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170615145224.66200-2-kirill.shutemov@linux.intel.com> Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: Andrew Morton , Vlastimil Babka , Vineet Gupta , Russell King , Will Deacon , Catalin Marinas , Ralf Baechle , "David S. Miller" , "Aneesh Kumar K . V" , Martin Schwidefsky , Heiko Carstens , linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Ingo Molnar , "H . Peter Anvin" , Thomas Gleixner Hello Krill, On Thu, Jun 15, 2017 at 05:52:22PM +0300, Kirill A. Shutemov wrote: > +static inline pmd_t pmdp_establish(pmd_t *pmdp, pmd_t pmd) > +{ > + pmd_t old; > + > + /* > + * We cannot assume what is value of pmd here, so there's no easy way > + * to set if half by half. We have to fall back to cmpxchg64. > + */ > + { > + old = *pmdp; > + } while (cmpxchg64(&pmdp->pmd, old.pmd, pmd.pmd) != old.pmd); > + > + return old; > +} I see further margin for optimization here (although it's only for PAE x32..). pmd is stable so we could do: if (!(pmd & _PAGE_PRESENT)) { cast to split_pmd and use xchg on pmd_low like native_pmdp_get_and_clear and copy pmd_high non atomically } else { the above cmpxchg64 loop } Now thinking about the above I had a second thought if pmdp_establish is the right interface and if we shouldn't replace pmdp_establish with pmdp_mknotpresent instead to skip the pmd & _PAGE_PRESENT check that will always be true in practice, so pmdp_mknotpresent will call internally pmd_mknotpresent and it won't have to check for pmd & _PAGE_PRESENT and it would have no cons on x86-64. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org