On Tuesday, 1 November 2016 00:40:29 GMT James Hogan wrote:
> When low memory doesn't reach HIGHMEM_START (e.g. up to 256MB at PA=0 is
> common) and highmem is present above HIGHMEM_START (e.g. on Malta the
> RAM overlayed by the IO region is aliased at PA=0x90000000), max_low_pfn
> will be initially calculated very large and then clipped down to
> HIGHMEM_START.
>
> This causes crashes when reading /sys/kernel/mm/page_idle/bitmap
> (i.e. CONFIG_IDLE_PAGE_TRACKING=y) when highmem is disabled. pfn_valid()
> will compare against max_mapnr which is derived from max_low_pfn when
> there is no highend_pfn set up, and will return true for PFNs right up
> to HIGHMEM_START, even though they are beyond the end of low memory and
> no page structs will actually exist for these PFNs.
>
> This is fixed by skipping high memory regions when initially calculating
> max_low_pfn if highmem is disabled, so it doesn't get clipped too high.
>
> Signed-off-by: James Hogan <james.hogan@xxxxxxxxxx>
> Cc: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
> Cc: linux-mips@xxxxxxxxxxxxxx
> ---
> arch/mips/kernel/setup.c | 9 +++++++++
> 1 file changed, 9 insertions(+), 0 deletions(-)
>
> diff --git a/arch/mips/kernel/setup.c b/arch/mips/kernel/setup.c
> index 0d57909d9026..cb6e5895bb7e 100644
> --- a/arch/mips/kernel/setup.c
> +++ b/arch/mips/kernel/setup.c
> @@ -368,6 +368,15 @@ static void __init bootmem_init(void)
> end = PFN_DOWN(boot_mem_map.map[i].addr
> + boot_mem_map.map[i].size);
>
> +#ifndef CONFIG_HIGHMEM
> + /*
> + * Skip highmem here so we get an accurate max_low_pfn if low
> + * memory stops short of high memory.
> + */
> + if (start >= PFN_DOWN(HIGHMEM_START))
> + continue;
> +#endif
> +
> if (end > max_low_pfn)
> max_low_pfn = end;
> if (start < min_low_pfn)
Hi James,
Shouldn't this also clip any region which crosses the boundary from lowmem to
highmem? (ie. start < PFN_DOWN(HIGHMEM_START) < end)
Thanks,
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