Re: [PATCH 1/2] arm64: kdump: simplify the reservation behaviour of crashkernel=,high

[Baoquan He]

On 01/20/23 at 10:04am, Simon Horman wrote:
> On Tue, Jan 17, 2023 at 11:49:20AM +0800, Baoquan He wrote:
> > On arm64, reservation for 'crashkernel=xM,high' is taken by searching for
> > suitable memory region up down. If the 'xM' of crashkernel high memory
> > is reserved from high memory successfully, it will try to reserve
> > crashkernel low memory later accoringly. Otherwise, it will try to search
> > low memory area for the 'xM' suitable region.
> > 
> > While we observed an unexpected case where a reserved region crosses the
> > high and low meomry boundary. E.g on a system with 4G as low memory end,
> > user added the kernel parameters like: 'crashkernel=512M,high', it could
> > finally have [4G-126M, 4G+386M], [1G, 1G+128M] regions in running kernel.
> > This looks very strange because we have two low memory regions
> > [4G-126M, 4G] and [1G, 1G+128M]. Much explanation need be given to tell
> > why that happened.
> > 
> > Here, for crashkernel=xM,high, search the high memory for the suitable
> > region above the high and low memory boundary. If failed, try reserving
> > the suitable region below the boundary. Like this, the crashkernel high
> > region will only exist in high memory, and crashkernel low region only
> > exists in low memory. The reservation behaviour for crashkernel=,high is
> > clearer and simpler.
> > 
> > Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
> > ---
> >  arch/arm64/mm/init.c | 30 +++++++++++++++++++++++-------
> >  1 file changed, 23 insertions(+), 7 deletions(-)
> > 
> > diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> > index 58a0bb2c17f1..26a05af2bfa8 100644
> > --- a/arch/arm64/mm/init.c
> > +++ b/arch/arm64/mm/init.c
> > @@ -127,12 +127,13 @@ static int __init reserve_crashkernel_low(unsigned long long low_size)
> >   */
> >  static void __init reserve_crashkernel(void)
> >  {
> > -	unsigned long long crash_base, crash_size;
> > -	unsigned long long crash_low_size = 0;
> > +	unsigned long long crash_base, crash_size, search_base;
> >  	unsigned long long crash_max = CRASH_ADDR_LOW_MAX;
> > +	unsigned long long crash_low_size = 0;
> >  	char *cmdline = boot_command_line;
> > -	int ret;
> >  	bool fixed_base = false;
> > +	bool high = false;
> > +	int ret;
> >  
> >  	if (!IS_ENABLED(CONFIG_KEXEC_CORE))
> >  		return;
> > @@ -155,7 +156,9 @@ static void __init reserve_crashkernel(void)
> >  		else if (ret)
> >  			return;
> >  
> > +		search_base = CRASH_ADDR_LOW_MAX;
> >  		crash_max = CRASH_ADDR_HIGH_MAX;
> > +		high = true;
> >  	} else if (ret || !crash_size) {
> >  		/* The specified value is invalid */
> >  		return;
> > @@ -166,31 +169,44 @@ static void __init reserve_crashkernel(void)
> >  	/* User specifies base address explicitly. */
> >  	if (crash_base) {
> >  		fixed_base = true;
> > +		search_base = crash_base;
> >  		crash_max = crash_base + crash_size;
> >  	}
> >  
> >  retry:
> >  	crash_base = memblock_phys_alloc_range(crash_size, CRASH_ALIGN,
> > -					       crash_base, crash_max);
> > +					       search_base, crash_max);
> >  	if (!crash_base) {
> > +		if (fixed_base) {
> > +			pr_warn("cannot reserve crashkernel region [0x%llx-0x%llx]\n",
> > +				search_base, crash_max);
> > +			return;
> > +		}
> > +
> >  		/*
> >  		 * If the first attempt was for low memory, fall back to
> >  		 * high memory, the minimum required low memory will be
> >  		 * reserved later.
> >  		 */
> > -		if (!fixed_base && (crash_max == CRASH_ADDR_LOW_MAX)) {
> > +		if (!high && crash_max == CRASH_ADDR_LOW_MAX) {
> >  			crash_max = CRASH_ADDR_HIGH_MAX;
> > +			search_base = CRASH_ADDR_LOW_MAX;
> >  			crash_low_size = DEFAULT_CRASH_KERNEL_LOW_SIZE;
> >  			goto retry;
> >  		}
> >  
> > +		if (high && (crash_max == CRASH_ADDR_HIGH_MAX)) {
> 
> nit: unnecessary (and inconsistent with code just above) parentheses.

Indeed, will remove it. Thanks for reviewing.

> 
> > +			crash_max = CRASH_ADDR_LOW_MAX;
> > +			search_base = 0;
> > +			goto retry;
> > +		}
> >  		pr_warn("cannot allocate crashkernel (size:0x%llx)\n",
> >  			crash_size);
> >  		return;
> >  	}
> >  
> > -	if ((crash_base > CRASH_ADDR_LOW_MAX - crash_low_size) &&
> > -	     crash_low_size && reserve_crashkernel_low(crash_low_size)) {
> > +	if ((crash_base >= CRASH_ADDR_LOW_MAX) && crash_low_size &&
> > +	     reserve_crashkernel_low(crash_low_size)) {
> >  		memblock_phys_free(crash_base, crash_size);
> >  		return;
> >  	}
> > -- 
> > 2.34.1
> > 
> > 
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