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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 4 Mar 2019 14:53:05 -0000 Received: from d06av21.portsmouth.uk.ibm.com (d06av21.portsmouth.uk.ibm.com [9.149.105.232]) by b06cxnps3074.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x24Er4qT59768974 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 4 Mar 2019 14:53:05 GMT Received: from d06av21.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id D00EC5204F; Mon, 4 Mar 2019 14:53:04 +0000 (GMT) Received: from rapoport-lnx (unknown [9.148.206.89]) by d06av21.portsmouth.uk.ibm.com (Postfix) with ESMTPS id 0AABA52063; Mon, 4 Mar 2019 14:53:02 +0000 (GMT) Date: Mon, 4 Mar 2019 16:53:01 +0200 From: Mike Rapoport To: Steven Price Cc: Mark Rutland , Dave Hansen , James Morse , Arnd Bergmann , Ard Biesheuvel , Peter Zijlstra , Catalin Marinas , x86@kernel.org, Will Deacon , linux-kernel@vger.kernel.org, linux-mm@kvack.org, =?iso-8859-1?B?Suly9G1l?= Glisse , Ingo Molnar , Borislav Petkov , Andy Lutomirski , "H. Peter Anvin" , "Kirill A. Shutemov" , Thomas Gleixner , linux-arm-kernel@lists.infradead.org, "Liang, Kan" Subject: Re: [PATCH v2 03/13] mm: Add generic p?d_large() macros References: <20190221142812.oa53lfnnfmsuh6ys@kshutemo-mobl1> <20190221145706.zqwfdoyiirn3lc7y@kshutemo-mobl1> <20190221210618.voyfs5cnafpvgedh@kshutemo-mobl1> <20190301115300.GE5156@rapoport-lnx> <20190301123031.rw3dswcoaa2x7haq@kshutemo-mobl1> <20190303071253.GA7585@rapoport-lnx> <2adbc516-3ffd-8e34-887a-843ccab72d51@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2adbc516-3ffd-8e34-887a-843ccab72d51@arm.com> User-Agent: Mutt/1.5.24 (2015-08-30) X-TM-AS-GCONF: 00 x-cbid: 19030414-0012-0000-0000-000002FE5B31 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19030414-0013-0000-0000-000021355E08 Message-Id: <20190304145300.GC22843@rapoport-lnx> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-03-04_06:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1903040109 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Mar 04, 2019 at 02:35:56PM +0000, Steven Price wrote: > On 03/03/2019 07:12, Mike Rapoport wrote: > > On Fri, Mar 01, 2019 at 01:39:30PM +0000, Steven Price wrote: > >> On 01/03/2019 12:30, Kirill A. Shutemov wrote: > >>> On Fri, Mar 01, 2019 at 01:53:01PM +0200, Mike Rapoport wrote: > >>>> Him Kirill, > >>>> > >>>> On Fri, Feb 22, 2019 at 12:06:18AM +0300, Kirill A. Shutemov wrote: > >>>>> On Thu, Feb 21, 2019 at 05:16:46PM +0000, Steven Price wrote: > >>>>>>>> Note that in terms of the new page walking code, these new defines are > >>>>>>>> only used when walking a page table without a VMA (which isn't currently > >>>>>>>> done), so architectures which don't use p?d_large currently will work > >>>>>>>> fine with the generic versions. They only need to provide meaningful > >>>>>>>> definitions when switching to use the walk-without-a-VMA functionality. > >>>>>>> > >>>>>>> How other architectures would know that they need to provide the helpers > >>>>>>> to get walk-without-a-VMA functionality? This looks very fragile to me. > >>>>>> > >>>>>> Yes, you've got a good point there. This would apply to the p?d_large > >>>>>> macros as well - any arch which (inadvertently) uses the generic version > >>>>>> is likely to be fragile/broken. > >>>>>> > >>>>>> I think probably the best option here is to scrap the generic versions > >>>>>> altogether and simply introduce a ARCH_HAS_PXD_LARGE config option which > >>>>>> would enable the new functionality to those arches that opt-in. Do you > >>>>>> think this would be less fragile? > >>>>> > >>>>> These helpers are useful beyond pagewalker. > >>>>> > >>>>> Can we actually do some grinding and make *all* archs to provide correct > >>>>> helpers? Yes, it's tedious, but not that bad. > >>>> > >>>> Many architectures simply cannot support non-leaf entries at the higher > >>>> levels. I think letting the use a generic helper actually does make sense. > >>> > >>> I disagree. > >>> > >>> It's makes sense if the level doesn't exists on the arch. > >> > >> This is what patch 24 [1] of the series does - if the level doesn't > >> exist then appropriate stubs are provided. > >> > >>> But if the level exists, it will be less frugile to ask the arch to > >>> provide the helper. Even if it is dummy always-false. > >> > >> The problem (as I see it), is we need a reliable set of p?d_large() > >> implementations to be able to walk arbitrary page tables. Either the > >> entire functionality of walking page tables without a VMA has to be an > >> opt-in per architecture, or we need to mandate that every architecture > >> provide these implementations. > > > > I agree that we need a reliable set of p?d_large(), but I'm still not > > convinced that every architecture should provide these. > > > > Why having generic versions if p?d_large() is more fragile, than e.g. > > p??__access_permitted() or atomic ops? > > Personally I feel having p?d_large implemented for each arch has the > following benefits: > > * Matches p?d_present/p?d_none/p?d_bad which all similarly have to be > implemented for all arches except for folded levels (when folded using > the generic code). > > * Gives the architecture maintainers a heads-up and an opportunity to > ensure that the implementations I've written are correct rather than > silently picking up the generic version. > > * When adding a new architecture it will be obvious that p?d_large > implementations are needed. > > The benefits of having a generic version seem to be: > > * No boiler plate for the architectures that don't support large pages > (saves a handful of lines). > > * Easier to merge (fewer patches). > > While the last one is certainly appealing (to me at least), I'm not > convinced the benefits of the generic version outweigh those of > providing implementations per-arch. > > Am I missing something? > > > IMHO, adding those functions/macros for architectures that support large > > pages and providing defines to avoid override of 'static inline' implementations > > would be robust enough and will avoid unnecessary stubs in architectures > > that don't have large pages. > > Clearly at run time there's no difference in the "robustness" - the code > generation should be the same. So it's purely down to development processes. > > However, if you prefer I can resurrect the generic versions and drop the > patches that simply add dummy implementations. My concern was the code duplication, which didn't seem necessary. It's not only about saving a handful of lines, but rather having as many of the code shared by different architectures actually shared and not copied. I'd really appreciate having the dummy versions in include/asm-generic rather than all over arch/*/include/asm. > Steve > -- Sincerely yours, Mike.