linux-kernel - Re: [PATCH] arm64: mm: Fix memmap to be initialized for the entire section

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Date:   Thu, 24 Nov 2016 14:51:51 +0100
From:   Robert Richter <robert.richter@...ium.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
CC:     Will Deacon <will.deacon@....com>,
        Robert Richter <rric@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        David Daney <david.daney@...ium.com>,
        Hanjun Guo <hanjun.guo@...aro.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arm64: mm: Fix memmap to be initialized for the entire
 section

On 24.11.16 13:44:31, Ard Biesheuvel wrote:
> On 24 November 2016 at 13:42, Robert Richter <robert.richter@...ium.com> wrote:
> > On 23.11.16 21:25:06, Ard Biesheuvel wrote:
> >> Why? MEMREMAP_WB is used often, among other things for mapping
> >> firmware tables, which are marked as NOMAP, so in these cases, the
> >> linear address is not mapped.
> >
> > If fw tables are mapped wb, that is wrong and needs a separate fix.
> >
> 
> Why is that wrong?

The whole issue with mapping acpi tables is not marking them cachable,
what wb does. Otherwise we could just use linear mapping for those mem
ranges.

> >> > If you think pfn_valid() is wrong here, I am happy to send a patch
> >> > that fixes this by using page_is_ram(). In any case, the worst case
> >> > that may happen is to behave the same as v4.4, we might fix then the
> >> > wrong use of pfn_valid() where it is not correctly used to check for
> >> > ram.
> >> >
> >>
> >> page_is_ram() uses string comparisons to look for regions called
> >> 'System RAM'. Is that something we can tolerate for each pfn_valid()
> >> calll?
> >>
> >> Perhaps the solution is to reimplement page_is_ram() for arm64 using
> >> memblock_is_memory() instead, But that still means we need to modify
> >> the generic memremap() code first to switch to it before changing the
> >> arm64 implementation of pfn_valid
> >
> > No, that's not the solution. pfn_valid() should just check if there is
> > a valid struct page, as other archs do. And if there is a mis-use of
> > pfn_valid() to check for ram, only that calls should be fixed to use
> > page_is_ram(), however this is implemented, or something appropriate.
> > But I don't see any problematic code, and if so, I will fix that.
> >
> 
> memremap() uses pfn_valid() to decide whether some address is covered
> by the linear mapping. If we correct pfn_valid() to adhere to your
> definition, we will need to fix memremap() first in any case.

As said, will fix that if needed. But I think the caller is wrong
then.

-Robert