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

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Date:   Thu, 24 Nov 2016 19:42:47 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Robert Richter <robert.richter@...ium.com>
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 November 2016 at 19:26, Robert Richter <robert.richter@...ium.com> wrote:
> Ard,
>
>> > >> On 24 November 2016 at 13:51, Robert Richter <robert.richter@...ium.com> wrote:
>> > >> > On 24.11.16 13:44:31, Ard Biesheuvel wrote:
>
>> > >> Regions containing firmware tables are owned by the firmware, and it
>> > >> is the firmware that tells us which memory attributes we are allowed
>> > >> to use. If those attributes include WB, it is perfectly legal to use a
>> > >> cacheable mapping. That does *not* mean they should be covered by the
>> > >> linear mapping. The linear mapping is read-write-non-exec, for
>> > >> instance, and we may prefer to use a read-only mapping and/or
>> > >> executable mapping.
>> > >
>> > > Ok, I am going to fix try_ram_remap().
>
> I revisited the code and it is working well already since:
>
>  e7cd190385d1 arm64: mark reserved memblock regions explicitly in iomem
>
> Now, try_ram_remap() is only called if the region to be mapped is
> entirely in IORESOURCE_SYSTEM_RAM. This is only true for normal mem
> ranges and not NOMAP mem. region_intersects() then returns
> REGION_INTERSECTS and calls try_ram_remap(). For the NOMAP memory case
> REGION_DISJOINT would be returned and thus arch_memremap_wb() being
> called directly. Before the e7cd190385d1 change try_ram_remap() was
> called also for nomap regions.
>
> So we can leave memremap() as it is and just apply this patch
> unmodified. What do you think?

I agree. The pfn_valid() check in try_ram_remap() is still appropriate
simply because the PageHighmem check requires a valid struct page. But
if we don't enter that code path anymore for NOMAP regions, I think
we're ok.

> Please ack.
>

I still don't fully understand how it is guaranteed that *all* memory
(i.e., all regions for which memblock_is_memory() returns true) is
covered by a struct page, but marked as reserved. Are we relying on
the fact that NOMAP memory is also memblock_reserve()'d?

> I am going to prepare the pfn_is_ram() change in addition to this
> patch, but that should not be required for this fix to work correcly.
>

I don't think you need to bother with page_is_ram() then. The only
place we use it is in devmem_is_allowed(), and there it makes sense to
allow NOMAP regions to be accessed (provided that you think /dev/mem
is a good idea in the first place).