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

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Message-Id: <6d454246-578f-4a25-bddf-2f68f32d2213@app.fastmail.com>
Date:   Wed, 19 Oct 2022 11:32:25 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Russell King" <linux@...linux.org.uk>
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 Wed, Oct 19, 2022, at 11:09, Russell King (Oracle) wrote:

> 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.

Fair enough.

> 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.

It should be easy enough to not expose it by renaming the
symbol to something other than empty_zero_page. That way,
any incorrect users that may come up in the future would
at least result in a build failure instead of runtime
data corruption.

> 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. :)

The SPI driver is powerpc specific, so it's also not going to
get hit. 

    Arnd