Subject: [PATCH v22 1/8] arm64: kdump: reserve memory for crash dump kernel

From: [email-masked] (AKASHI Takahiro)
Date: Wed, 20 Jul 2016 11:17:23 +0900

On Tue, Jul 19, 2016 at 02:27:54PM +0100, Mark Rutland wrote:
> On Tue, Jul 19, 2016 at 08:48:57AM -0400, Mark Salter wrote:
> > On Tue, 2016-07-19 at 18:41 +0800, Dennis Chen wrote:
> > > On Tue, Jul 19, 2016 at 07:28:16PM +0900, AKASHI Takahiro wrote:
> > > > On Tue, Jul 19, 2016 at 05:39:07PM +0800, Dennis Chen wrote:
> > > > > On Tue, Jul 12, 2016 at 02:05:07PM +0900, AKASHI Takahiro wrote:
> > > > > > +/*
> > > > > > + * reserve_crashkernel() - reserves memory for crash kernel
> > > > > > + *
> > > > > > + * This function reserves memory area given in "crashkernel=" kernel command
> > > > > > + * line parameter. The memory reserved is used by dump capture kernel when
> > > > > > + * primary kernel is crashing.
> > > > > > + */
> > > > > > +static void __init reserve_crashkernel(void)
> > > > > > +{
> > > > > > +??????????int ret;
> > > > > > +
> > > > > > +??????????ret = parse_crashkernel(boot_command_line, memblock_phys_mem_size(),
> > > > > > +??????????????????????????????????????????????????????????&crash_size, &crash_base);
> > > > > > +??????????/* no crashkernel= or invalid value specified */
> > > > > > +??????????if (ret || !crash_size)
> > > > > > +??????????????????????????return;
> > > > > > +
> > > > > > +??????????if (crash_base == 0) {
> > > > > > +??????????????????????????/* Current arm64 boot protocol requires 2MB alignment */
> > > > > > +??????????????????????????crash_base = memblock_find_in_range(0,
> > > > > > +??????????????????????????????????????????????????????????MEMBLOCK_ALLOC_ACCESSIBLE, crash_size, SZ_2M);
> > > > > > +??????????????????????????if (crash_base == 0) {
> > > > > > +??????????????????????????????????????????pr_warn("Unable to allocate crashkernel (size:%llx)\n",
> > > > > > +??????????????????????????????????????????????????????????crash_size);
> > > > > > +??????????????????????????????????????????return;
> > > > > > +??????????????????????????}
> > > > > > +??????????????????????????memblock_reserve(crash_base, crash_size);
> > > > > > 
> > > > > I am not pretty sure the context here, but
> > > > > can we use below code piece instead of the above lines?
> > > > > ????????????????if (crash_base == 0)
> > > > > ????????????????????????????????memblock_alloc(crash_size, SZ_2M);
> > > > Either would be fine here.
> > > > 
> > > Hello AKASHI, maybe you can succeed to find the base with memblock_find_in_range(),??
> > > but that doesn't mean you will also succeed to reserve them with memblock_reserve followed.
> > 
> > We avoid memblock_alloc() here because it panics on failure. This could happen
> > if user asks for an unusually large crashkernel size. Better to print a message
> > and keep booting. Checking the return value of memblock_reserve() seems like a
> > good thing to do though.
> 
> Another option would be to add a memblock_try_alloc() function to the
> memblock API, which in case of failure returns 0 rather than triggering
> a panic(). We'd still have to check the return value, but all the
> memblock manipulation would be in one place.
> 
> It looks like adding that is fairly simple:
> 
> phys_addr_t __init memblock_try_alloc(phys_addr_t size, phys_addr_t align)
> {
> 	return __memblock_alloc_base(size, align, MEMBLOCK_ALLOC_ACCESSIBLE);
> }

Apart form this issue, I found that the kernel can panic, if the memory
for crash dump kernel is allocated above 32-bits, due to a failure
of alloc_bootmem_low() in request_standard_resources().
(In addition, dma_contiguous_reserve() will also fail.)

So I'd like to change the code to:
        if (crash_base == 0) {
                crash_base = memblock_find_in_range(0,
                                IS_ENABLED(CONFIG_ZONE_DMA) ?
                                        ARCH_LOW_ADDRESS_LIMIT :
                                        MEMBLOCK_ALLOC_ACCESSIBLE,
                                crash_size, SZ_2M);
        ...

Does this make sense?

Thanks,
-Takahiro AKASHI

> Thanks,
> Mark.