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

From: [email-masked] (Dennis Chen)
Date: Wed, 20 Jul 2016 11:48:40 +0800

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);
> }
>
Hi Mark, memblock_alloc() should remove the panic just as we did in kmalloc IMO, as a memory allocation
API, it should handover the failure case to the user and the latter should make the final decision.
Just take a look at the usage case of the memblock_alloc() in the kernel tree, codes have been messed
up by -- some codes check the return value of memblock_alloc, while others not.

Thanks,
Dennis 
> Thanks,
> Mark.