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

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Message-Id: <dc5a5c40-8e96-4f91-a3c6-5a1fc8b26ad1@app.fastmail.com>
Date:   Tue, 18 Oct 2022 20:35:36 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Giulio Benetti" <giulio.benetti@...ettiengineering.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     "Russell King" <linux@...linux.org.uk>,
        "Anshuman Khandual" <anshuman.khandual@....com>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Kefeng Wang" <wangkefeng.wang@...wei.com>,
        "Russell King" <rmk+kernel@...linux.org.uk>,
        "Will Deacon" <will@...nel.org>
Subject: Re: [PATCH] ARM: mm: fix no-MMU ZERO_PAGE() implementation

On Tue, Oct 18, 2022, at 19:44, Giulio Benetti wrote:
> On 18/10/22 09:03, Arnd Bergmann wrote:
>> On Tue, Oct 18, 2022, at 1:37 AM, Giulio Benetti wrote:
>>> Actually in no-MMU SoCs(i.e. i.MXRT) ZERO_PAGE(vaddr) expands to
>
>> It looks like we dropped the ball on this when it came up last.
>> I'm also not sure when we started requiring this, any idea?
>
> No to be honest. But in my case I've met ZERO_PAGE() calling in sdhci
> driver. And as stated on the ML link above:
> ```
> But I wonder if it's safe for noMMU architectures to go on without a
> working ZERO_PAGE(0).  It has uses scattered throughout the tree, in
> drivers, fs, crypto and more, and it's not at all obvious (to me) that
> they all depend on CONFIG_MMU.
> ```
> And I've found this driver that requires it and probably is not the last
> since imxrt support is not complete.
>
>> I can see that microblaze-nommu used BUG() in ZERO_PAGE(), so at
>> whenever microblaze last worked, we clearly did not call it.
>
> This probably means that microblaze-nommu doesn't use drivers or other
> subsystems that require ZERO_PAGE().

To clarify: microblaze-nommu support was removed two years ago,
and probably was already broken for a while before that.

>> 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.
>
> I was about doing it, but then I tought to move one piece at a time.

Right, it would definitely be a separate patch, but it
can be a series of two patches. We probably wouldn't need to
backport the second patch that turns it into a static allocation.

> But yes, I can modify accordingly. That way we also avoid the early
> allocation in pagint_init() since it would be a .bss array.

>> There are three references to empty_zero_page in architecture
>> independent code, and while we don't seem to use any of them
>> on Arm, they would clearly be wrong if we did:
>> 
>> drivers/acpi/scan.c:#define INVALID_ACPI_HANDLE ((acpi_handle)empty_zero_page)
>> drivers/spi/spi-fsl-cpm.c:      mspi->dma_dummy_tx = dma_map_single(dev, empty_zero_page, PAGE_SIZE,
>> include/linux/raid/pq.h:# define raid6_empty_zero_page empty_zero_page
>
> For them I can send patches to substitute with PAGE_ZERO(0) correctly 
> adapted.
>
> What do you think?

That sounds like a good idea as well.

     Arnd