From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39968 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731323AbgIJPsO (ORCPT ); Thu, 10 Sep 2020 11:48:14 -0400 Received: from mail-qk1-x741.google.com (mail-qk1-x741.google.com [IPv6:2607:f8b0:4864:20::741]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8417AC06135C for ; Thu, 10 Sep 2020 08:10:29 -0700 (PDT) Received: by mail-qk1-x741.google.com with SMTP id w16so6389908qkj.7 for ; Thu, 10 Sep 2020 08:10:29 -0700 (PDT) Date: Thu, 10 Sep 2020 12:10:26 -0300 From: Jason Gunthorpe Subject: Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table folding Message-ID: <20200910151026.GL87483@ziepe.ca> References: <20200907180058.64880-1-gerald.schaefer@linux.ibm.com> <20200907180058.64880-2-gerald.schaefer@linux.ibm.com> <0dbc6ec8-45ea-0853-4856-2bc1e661a5a5@intel.com> <20200909142904.00b72921@thinkpad> <20200909192534.442f8984@thinkpad> <20200909180324.GI87483@ziepe.ca> <20200910093925.GB29166@oc3871087118.ibm.com> <20200910130233.GK87483@ziepe.ca> <20200910152803.1a930afc@thinkpad> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200910152803.1a930afc@thinkpad> Sender: linux-s390-owner@vger.kernel.org List-ID: To: Gerald Schaefer , Anshuman Khandual Cc: Alexander Gordeev , Dave Hansen , John Hubbard , LKML , linux-mm , linux-arch , Andrew Morton , Linus Torvalds , Russell King , Mike Rapoport , Catalin Marinas , Will Deacon , Michael Ellerman , Benjamin Herrenschmidt , Paul Mackerras , Jeff Dike , Richard Weinberger , Dave Hansen , Andy Lutomirski , Peter Zijlstra , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Arnd Bergmann , Andrey Ryabinin , linux-x86 , linux-arm , linux-power , linux-sparc , linux-um , linux-s390 , Vasily Gorbik , Heiko Carstens , Christian Borntraeger , Claudio Imbrenda On Thu, Sep 10, 2020 at 03:28:03PM +0200, Gerald Schaefer wrote: > On Thu, 10 Sep 2020 10:02:33 -0300 > Jason Gunthorpe wrote: > > > On Thu, Sep 10, 2020 at 11:39:25AM +0200, Alexander Gordeev wrote: > > > > > As Gerald mentioned, it is very difficult to explain in a clear way. > > > Hopefully, one could make sense ot of it. > > > > I would say the page table API requires this invariant: > > > > pud = pud_offset(p4d, addr); > > do { > > WARN_ON(pud != pud_offset(p4d, addr); > > next = pud_addr_end(addr, end); > > } while (pud++, addr = next, addr != end); > > > > ie pud++ is supposed to be a shortcut for > > pud_offset(p4d, next) > > > > While S390 does not follow this. Fixing addr_end brings it into > > alignment by preventing pud++ from happening. > > > > The only currently known side effect is that gup_fast crashes, but it > > sure is an unexpected thing. > > It only is unexpected in a "top-level folding" world, see my other reply. > Consider it an optimization, which was possible because of how our dynamic > folding works, and e.g. because we can determine the correct pagetable > level from a pXd value in pXd_offset. No, I disagree. The page walker API the arch presents has to have well defined semantics. For instance, there is an effort to define tests and invarients for the page table accesses to bring this understanding and uniformity: mm/debug_vm_pgtable.c If we fix S390 using the pX_addr_end() change then the above should be updated with an invariant to check it. I've added Anshuman for some thoughts.. For better or worse, that invariant does exclude arches from using other folding techniques. The other solution would be to address the other side of != and adjust the pud++ eg replcae pud++ with something like: pud = pud_next_entry(p4d, pud, next) Such that: pud_next_entry(p4d, pud, next) === pud_offset(p4d, next) In which case the invarient changes to 'callers can never do pointer arithmetic on the result of pXX_offset()' which is a bit harder to enforce. Jason