linux-kernel - Re: [PATCH] ARM: mm: fix no-MMU ZERO

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Message-ID: <Y0++52o7QSaDvqpM@shell.armlinux.org.uk>
Date:   Wed, 19 Oct 2022 10:09:59 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Giulio Benetti <giulio.benetti@...ettiengineering.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Anshuman Khandual <anshuman.khandual@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH] ARM: mm: fix no-MMU ZERO_PAGE() implementation

On Tue, Oct 18, 2022 at 09:03:01AM +0200, Arnd Bergmann wrote:
> In addition to your fix, I see that arm is the only architecture
> that defines 'empty_zero_page' as a pointer to the page, when
> everything else just makes it a pointer to the data itself,
> or an 'extern char empty_zero_page[]' array, which we may want
> to change for consistency.

ARM's implementation is the utterly sensible implementation IMHO.

When the only users in the kernel _were_ ZERO_PAGE() for this, which
is defined to return a struct page pointer, there was no need to make
"empty_zero_page" anything but a struct page pointer, rather than a
runtime translation from an address to a struct page.

IMHO, we should _not_ be exposing empty_zero_page to devices - we
certainly do not want the DMA API performing cache maintenance on
this page since the primary purpose of this page is to fill in
userspace BSS pages that have not been written.

ACPI's use is just to have a cookie for invalid handles, and using
the struct page pointer is good enough.

The only problem one is the RAID6 code, but that is disabled:

/* Set to 1 to use kernel-wide empty_zero_page */
#define RAID6_USE_EMPTY_ZERO_PAGE 0

#if RAID6_USE_EMPTY_ZERO_PAGE
# define raid6_empty_zero_page empty_zero_page
#else
extern const char raid6_empty_zero_page[PAGE_SIZE];
#endif

So, the only one that needs fixing is the SPI usage, which IMHO
is wrong. ARM being different finds what I consider a driver bug.
Good for 32-bit ARM. :)

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