Re: [PATCH 1/2] pstore-ram: Fix hangs by using write-combine mappings

[Kees Cook]

On Fri, Sep 12, 2014 at 11:32 AM, Tony Lindgren <tony@xxxxxxxxxxx> wrote:
> From: Rob Herring <robherring2@xxxxxxxxx>
>
> Currently trying to use pstore on at least ARMs can hang as we're
> mapping the peristent RAM with pgprot_noncached().
>
> On ARMs, pgprot_noncached() will actually make the memory strongly
> ordered, and as the atomic operations pstore uses are implementation
> defined for strongly ordered memory, they may not work. So basically
> atomic operations have undefined behavior on ARM for device or strongly
> ordered memory types.
>
> Let's fix the issue by using write-combine variants for mappings. This
> corresponds to normal, non-cacheable memory on ARM. For many other
> architectures, this change does not change the mapping type as by
> default we have:
>
> #define pgprot_writecombine pgprot_noncached
>
> The reason why pgprot_noncached() was originaly used for pstore
> is because Colin Cross <ccross@xxxxxxxxxxx> had observed lost
> debug prints right before a device hanging write operation on some
> systems. For the platforms supporting pgprot_noncached(), we can
> add a an optional configuration option to support that. But let's
> get pstore working first before adding new features.
>
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Cc: Anton Vorontsov <cbouatmailru@xxxxxxxxx>
> Cc: Colin Cross <ccross@xxxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Olof Johansson <olof@xxxxxxxxx>
> Cc: Tony Luck <tony.luck@xxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Rob Herring <rob.herring@xxxxxxxxxxx>
> [tony@xxxxxxxxxxx: updated description]
> Signed-off-by: Tony Lindgren <tony@xxxxxxxxxxx>

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

Please Cc: stable@xxxxxxxxxxxxxxx on this one too.

Thanks!

-Kees

> ---
>  fs/pstore/ram_core.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/fs/pstore/ram_core.c b/fs/pstore/ram_core.c
> index 9d7b9a8..24f94b0 100644
> --- a/fs/pstore/ram_core.c
> +++ b/fs/pstore/ram_core.c
> @@ -392,7 +392,7 @@ static void *persistent_ram_vmap(phys_addr_t start, size_t size)
>         page_start = start - offset_in_page(start);
>         page_count = DIV_ROUND_UP(size + offset_in_page(start), PAGE_SIZE);
>
> -       prot = pgprot_noncached(PAGE_KERNEL);
> +       prot = pgprot_writecombine(PAGE_KERNEL);
>
>         pages = kmalloc_array(page_count, sizeof(struct page *), GFP_KERNEL);
>         if (!pages) {
> @@ -422,7 +422,7 @@ static void *persistent_ram_iomap(phys_addr_t start, size_t size)
>         buffer_start_add = buffer_start_add_locked;
>         buffer_size_add = buffer_size_add_locked;
>
> -       return ioremap(start, size);
> +       return ioremap_wc(start, size);
>  }
>
>  static int persistent_ram_buffer_map(phys_addr_t start, phys_addr_t size,
> --
> 2.1.0
>



-- 
Kees Cook
Chrome OS Security
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