Re: [PATCH 2/3] arm64: kdump: support more than one crash kernel regions

[Mike Rapoport <rppt@linux.ibm.com>]

Hi,

On Fri, Apr 05, 2019 at 11:47:27AM +0800, Chen Zhou wrote:
> Hi Mike,
> 
> On 2019/4/5 10:17, Chen Zhou wrote:
> > Hi Mike,
> > 
> > On 2019/4/4 22:44, Mike Rapoport wrote:
> >> Hi,
> >>
> >> On Wed, Apr 03, 2019 at 09:51:27PM +0800, Chen Zhou wrote:
> >>> Hi Mike,
> >>>
> >>> On 2019/4/3 19:29, Mike Rapoport wrote:
> >>>> On Wed, Apr 03, 2019 at 11:05:45AM +0800, Chen Zhou wrote:
> >>>>> After commit (arm64: kdump: support reserving crashkernel above 4G),
> >>>>> there may be two crash kernel regions, one is below 4G, the other is
> >>>>> above 4G.
> >>>>>
> >>>>> Crash dump kernel reads more than one crash kernel regions via a dtb
> >>>>> property under node /chosen,
> >>>>> linux,usable-memory-range = <BASE1 SIZE1 [BASE2 SIZE2]>
> >>>>>
> >>>>> Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
> >>>>> ---
> >>>>>  arch/arm64/mm/init.c     | 37 +++++++++++++++++++++++++------------
> >>>>>  include/linux/memblock.h |  1 +
> >>>>>  mm/memblock.c            | 40 ++++++++++++++++++++++++++++++++++++++++
> >>>>>  3 files changed, 66 insertions(+), 12 deletions(-)
> >>>>>
> >>>>> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> >>>>> index ceb2a25..769c77a 100644
> >>>>> --- a/arch/arm64/mm/init.c
> >>>>> +++ b/arch/arm64/mm/init.c
> >>>>> @@ -64,6 +64,8 @@ EXPORT_SYMBOL(memstart_addr);
> >>>>>  phys_addr_t arm64_dma_phys_limit __ro_after_init;
> >>>>>  
> >>>>>  #ifdef CONFIG_KEXEC_CORE
> >>>>> +# define CRASH_MAX_USABLE_RANGES        2
> >>>>> +
> >>>>>  static int __init reserve_crashkernel_low(void)
> >>>>>  {
> >>>>>  	unsigned long long base, low_base = 0, low_size = 0;
> >>>>> @@ -346,8 +348,8 @@ static int __init early_init_dt_scan_usablemem(unsigned long node,
> >>>>>  		const char *uname, int depth, void *data)
> >>>>>  {
> >>>>>  	struct memblock_region *usablemem = data;
> >>>>> -	const __be32 *reg;
> >>>>> -	int len;
> >>>>> +	const __be32 *reg, *endp;
> >>>>> +	int len, nr = 0;
> >>>>>  
> >>>>>  	if (depth != 1 || strcmp(uname, "chosen") != 0)
> >>>>>  		return 0;
> >>>>> @@ -356,22 +358,33 @@ static int __init early_init_dt_scan_usablemem(unsigned long node,
> >>>>>  	if (!reg || (len < (dt_root_addr_cells + dt_root_size_cells)))
> >>>>>  		return 1;
> >>>>>  
> >>>>> -	usablemem->base = dt_mem_next_cell(dt_root_addr_cells, &reg);
> >>>>> -	usablemem->size = dt_mem_next_cell(dt_root_size_cells, &reg);
> >>>>> +	endp = reg + (len / sizeof(__be32));
> >>>>> +	while ((endp - reg) >= (dt_root_addr_cells + dt_root_size_cells)) {
> >>>>> +		usablemem[nr].base = dt_mem_next_cell(dt_root_addr_cells, &reg);
> >>>>> +		usablemem[nr].size = dt_mem_next_cell(dt_root_size_cells, &reg);
> >>>>> +
> >>>>> +		if (++nr >= CRASH_MAX_USABLE_RANGES)
> >>>>> +			break;
> >>>>> +	}
> >>>>>  
> >>>>>  	return 1;
> >>>>>  }
> >>>>>  
> >>>>>  static void __init fdt_enforce_memory_region(void)
> >>>>>  {
> >>>>> -	struct memblock_region reg = {
> >>>>> -		.size = 0,
> >>>>> -	};
> >>>>> -
> >>>>> -	of_scan_flat_dt(early_init_dt_scan_usablemem, &reg);
> >>>>> -
> >>>>> -	if (reg.size)
> >>>>> -		memblock_cap_memory_range(reg.base, reg.size);
> >>>>> +	int i, cnt = 0;
> >>>>> +	struct memblock_region regs[CRASH_MAX_USABLE_RANGES];
> >>>>> +
> >>>>> +	memset(regs, 0, sizeof(regs));
> >>>>> +	of_scan_flat_dt(early_init_dt_scan_usablemem, regs);
> >>>>> +
> >>>>> +	for (i = 0; i < CRASH_MAX_USABLE_RANGES; i++)
> >>>>> +		if (regs[i].size)
> >>>>> +			cnt++;
> >>>>> +		else
> >>>>> +			break;
> >>>>> +	if (cnt)
> >>>>> +		memblock_cap_memory_ranges(regs, cnt);
> >>>>
> >>>> Why not simply call memblock_cap_memory_range() for each region?
> >>>
> >>> Function memblock_cap_memory_range() removes all memory type ranges except specified range.
> >>> So if we call memblock_cap_memory_range() for each region simply, there will be no usable-memory
> >>> on kdump capture kernel.
> >>
> >> Thanks for the clarification.
> >> I still think that memblock_cap_memory_ranges() is overly complex. 
> >>
> >> How about doing something like this:
> >>
> >> Cap the memory range for [min(regs[*].start, max(regs[*].end)] and then
> >> removing the range in the middle?
> > 
> > Yes, that would be ok. But that would do one more memblock_cap_memory_range operation.
> > That is, if there are n regions, we need to do (n + 1) operations, which doesn't seem to
> > matter.
> > 
> > I agree with you, your idea is better.
> > 
> > Thanks,
> > Chen Zhou
> 
> Sorry, just ignore my previous reply, I got that wrong.
> 
> I think it carefully, we can cap the memory range for [min(regs[*].start, max(regs[*].end)]
> firstly. But how to remove the middle ranges, we still can't use memblock_cap_memory_range()
> directly and the extra remove operation may be complex.
> 
> For more than one regions, i think add a new memblock_cap_memory_ranges() may be better.
> Besides, memblock_cap_memory_ranges() is also applicable for one region.
> 
> How about replace memblock_cap_memory_range() with memblock_cap_memory_ranges()?

arm64 is the only user of both MEMBLOCK_NOMAP and memblock_cap_memory_range()
and I don't expect other architectures will use these interfaces.
It seems that capping the memory for arm64 crash kernel the way I've
suggested can be implemented in fdt_enforce_memory_region(). If we'd ever
need such functionality elsewhere or CRASH_MAX_USABLE_RANGES will need to
grow we'll rethink the solution.
 
> Thanks,
> Chen Zhou

-- 
Sincerely yours,
Mike.


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