From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vineet Gupta Subject: Re: [PATCH 0/3] eldie generated code for folded p4d/pud Date: Thu, 10 Oct 2019 13:05:56 -0700 Message-ID: <8ba067a6-8b6a-2414-0f04-b251cd6bb47c@gmail.com> References: <20191009222658.961-1-vgupta@synopsys.com> <20191010085609.xgwkrbzea253wmfg@black.fi.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Return-path: In-Reply-To: <20191010085609.xgwkrbzea253wmfg@black.fi.intel.com> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: "Kirill A. Shutemov" , Vineet Gupta Cc: linux-arch@vger.kernel.org, Arnd Bergmann , Peter Zijlstra , "Aneesh Kumar K . V" , linux-kernel@vger.kernel.org, Nick Piggin , linux-mm@kvack.org, Andrew Morton , linux-snps-arc@lists.infradead.org, Will Deacon List-Id: linux-arch.vger.kernel.org Hi Kirill, On 10/10/19 1:56 AM, Kirill A. Shutemov wrote: > On Wed, Oct 09, 2019 at 10:26:55PM +0000, Vineet Gupta wrote: >> >> This series elides extraneous generate code for folded p4d/pud. >> This came up when trying to remove __ARCH_USE_5LEVEL_HACK from ARC port. >> The code saving are not a while lot, but still worthwhile IMHO. > > Agreed. Thx. So given we are folding pmd too, it seemed we could do the following as well. +#ifndef __PAGETABLE_PMD_FOLDED void pmd_clear_bad(pmd_t *); +#else +#define pmd_clear_bad(pmd) do { } while (0) +#endif +#ifndef __PAGETABLE_PMD_FOLDED void pmd_clear_bad(pmd_t *pmd) { pmd_ERROR(*pmd); pmd_clear(pmd); } +#endif I stared at generated code and it seems a bit wrong. free_pgd_range() -> pgd_none_or_clear_bad() is no longer checking for unmapped pgd entries as pgd_none/pgd_bad are all stubs returning 0. This whole pmd folding is a bit confusing considering I only revisit it every few years :-) Abstraction wise, __PAGETABLE_PMD_FOLDED only has pgd, pte but even in this regime bunch of pmd macros are still valid pmd_set(pmdp, ptep) { *pmdp.pud.p4d.pgd = (unsigned long)ptep } Is there a better way to make a mental model of this code folding. In an ideal world pmd folded would have meant pmd_* routines just vanish - poof. So in that sense I like your implementation under #[45]LEVEL_HACK where the level simply vanishes by code like #define p4d_t pgd_t. Perhaps there is lot of historic baggage, proliferated into arch code so hard to untangle. Thx, -Vineet From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg1-f196.google.com ([209.85.215.196]:41804 "EHLO mail-pg1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725867AbfJJUGB (ORCPT ); Thu, 10 Oct 2019 16:06:01 -0400 Subject: Re: [PATCH 0/3] eldie generated code for folded p4d/pud References: <20191009222658.961-1-vgupta@synopsys.com> <20191010085609.xgwkrbzea253wmfg@black.fi.intel.com> From: Vineet Gupta Message-ID: <8ba067a6-8b6a-2414-0f04-b251cd6bb47c@gmail.com> Date: Thu, 10 Oct 2019 13:05:56 -0700 MIME-Version: 1.0 In-Reply-To: <20191010085609.xgwkrbzea253wmfg@black.fi.intel.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-arch-owner@vger.kernel.org List-ID: To: "Kirill A. Shutemov" , Vineet Gupta Cc: linux-arch@vger.kernel.org, Arnd Bergmann , Peter Zijlstra , "Aneesh Kumar K . V" , linux-kernel@vger.kernel.org, Nick Piggin , linux-mm@kvack.org, Andrew Morton , linux-snps-arc@lists.infradead.org, Will Deacon Message-ID: <20191010200556.5lRTpVmQNlj9K0z8OcbAtau2lie9pJjfqp8LyVj9_-Q@z> Hi Kirill, On 10/10/19 1:56 AM, Kirill A. Shutemov wrote: > On Wed, Oct 09, 2019 at 10:26:55PM +0000, Vineet Gupta wrote: >> >> This series elides extraneous generate code for folded p4d/pud. >> This came up when trying to remove __ARCH_USE_5LEVEL_HACK from ARC port. >> The code saving are not a while lot, but still worthwhile IMHO. > > Agreed. Thx. So given we are folding pmd too, it seemed we could do the following as well. +#ifndef __PAGETABLE_PMD_FOLDED void pmd_clear_bad(pmd_t *); +#else +#define pmd_clear_bad(pmd) do { } while (0) +#endif +#ifndef __PAGETABLE_PMD_FOLDED void pmd_clear_bad(pmd_t *pmd) { pmd_ERROR(*pmd); pmd_clear(pmd); } +#endif I stared at generated code and it seems a bit wrong. free_pgd_range() -> pgd_none_or_clear_bad() is no longer checking for unmapped pgd entries as pgd_none/pgd_bad are all stubs returning 0. This whole pmd folding is a bit confusing considering I only revisit it every few years :-) Abstraction wise, __PAGETABLE_PMD_FOLDED only has pgd, pte but even in this regime bunch of pmd macros are still valid pmd_set(pmdp, ptep) { *pmdp.pud.p4d.pgd = (unsigned long)ptep } Is there a better way to make a mental model of this code folding. In an ideal world pmd folded would have meant pmd_* routines just vanish - poof. So in that sense I like your implementation under #[45]LEVEL_HACK where the level simply vanishes by code like #define p4d_t pgd_t. Perhaps there is lot of historic baggage, proliferated into arch code so hard to untangle. Thx, -Vineet