linux-kernel - Re: [PATCH V3 09/30] arm/mm: Enable ARCH_HAS_VM_GET_PAGE

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Message-ID: <Yhyqjo/4bozJB6j5@shell.armlinux.org.uk>
Date:   Mon, 28 Feb 2022 10:57:18 +0000
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, geert@...ux-m68k.org,
        Christoph Hellwig <hch@...radead.org>,
        linuxppc-dev@...ts.ozlabs.org,
        linux-arm-kernel@...ts.infradead.org, sparclinux@...r.kernel.org,
        linux-mips@...r.kernel.org, linux-m68k@...ts.linux-m68k.org,
        linux-s390@...r.kernel.org, linux-riscv@...ts.infradead.org,
        linux-alpha@...r.kernel.org, linux-sh@...r.kernel.org,
        linux-snps-arc@...ts.infradead.org, linux-csky@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, linux-parisc@...r.kernel.org,
        openrisc@...ts.librecores.org, linux-um@...ts.infradead.org,
        linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-arch@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH V3 09/30] arm/mm: Enable ARCH_HAS_VM_GET_PAGE_PROT

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.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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