From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anshuman Khandual Date: Tue, 01 Mar 2022 00:12:41 +0000 Subject: Re: [PATCH V3 09/30] arm/mm: Enable ARCH_HAS_VM_GET_PAGE_PROT Message-Id: <542fa048-131e-240b-cc3a-fd4fff7ce4ba@arm.com> List-Id: References: <1646045273-9343-1-git-send-email-anshuman.khandual@arm.com> <1646045273-9343-10-git-send-email-anshuman.khandual@arm.com> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: "Russell King (Oracle)" Cc: linux-mm@kvack.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, geert@linux-m68k.org, Christoph Hellwig , linuxppc-dev@lists.ozlabs.org, linux-arm-kernel@lists.infradead.org, sparclinux@vger.kernel.org, linux-mips@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-s390@vger.kernel.org, linux-riscv@lists.infradead.org, linux-alpha@vger.kernel.org, linux-sh@vger.kernel.org, linux-snps-arc@lists.infradead.org, linux-csky@vger.kernel.org, linux-xtensa@linux-xtensa.org, linux-parisc@vger.kernel.org, openrisc@lists.librecores.org, linux-um@lists.infradead.org, linux-hexagon@vger.kernel.org, linux-ia64@vger.kernel.org, linux-arch@vger.kernel.org, Arnd Bergmann On 2/28/22 4:27 PM, Russell King (Oracle) wrote: > On Mon, Feb 28, 2022 at 04:17:32PM +0530, Anshuman Khandual wrote: >> This defines and exports a platform specific custom vm_get_page_prot() via >> subscribing ARCH_HAS_VM_GET_PAGE_PROT. Subsequently all __SXXX and __PXXX >> macros can be dropped which are no longer needed. > > What I would really like to know is why having to run _code_ to work out > what the page protections need to be is better than looking it up in a > table. > > Not only is this more expensive in terms of CPU cycles, it also brings > additional code size with it. > > I'm struggling to see what the benefit is. > Currently vm_get_page_prot() is also being _run_ to fetch required page protection values. Although that is being run in the core MM and from a platform perspective __SXXX, __PXXX are just being exported for a table. Looking it up in a table (and applying more constructs there after) is not much different than a clean switch case statement in terms of CPU usage. So this is not more expensive in terms of CPU cycles. -------------------------- pgprot_t protection_map[16] __ro_after_init = { __P000, __P001, __P010, __P011, __P100, __P101, __P110, __P111, __S000, __S001, __S010, __S011, __S100, __S101, __S110, __S111 }; #ifndef CONFIG_ARCH_HAS_FILTER_PGPROT static inline pgprot_t arch_filter_pgprot(pgprot_t prot) { return prot; } #endif pgprot_t vm_get_page_prot(unsigned long vm_flags) { pgprot_t ret = __pgprot(pgprot_val(protection_map[vm_flags & (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)]) | pgprot_val(arch_vm_get_page_prot(vm_flags))); return arch_filter_pgprot(ret); } EXPORT_SYMBOL(vm_get_page_prot) ---------------------------- There will be a single vm_get_page_prot() instance on a given platform just like before. So this also does not bring any additional code size with it. As mentioned earlier on a previous version. Remove multiple 'core MM <--> platform' abstraction layers to map vm_flags access permission combination into page protection. >From the cover letter ...... ---------- Currently there are multiple layers of abstraction i.e __SXXX/__PXXX macros , protection_map[], arch_vm_get_page_prot() and arch_filter_pgprot() built between the platform and generic MM, finally defining vm_get_page_prot(). Hence this series proposes to drop all these abstraction levels and instead just move the responsibility of defining vm_get_page_prot() to the platform itself making it clean and simple. ---------- Benefits 1. For platforms using arch_vm_get_page_prot() and/or arch_filter_pgprot() - A simplified vm_get_page_prot() - Dropped arch_vm_get_page_prot() and arch_filter_pgprot() - Dropped __SXXX, __PXXX macros 2. For platforms which just exported __SXXX, __PXXX - A simplified vm_get_page_prot() - Dropped __SXXX, __PXXX macros 3. For core MM - Dropped a complex vm_get_page_prot() with multiple layers of abstraction i.e __SXXX/__PXXX macros, protection_map[], arch_vm_get_page_prot(), arch_filter_pgprot() etc. - Anshuman