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

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Message-ID: <542fa048-131e-240b-cc3a-fd4fff7ce4ba@arm.com>
Date:   Tue, 1 Mar 2022 05:30:41 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
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 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