Re: [PATCH 5/5] arm64: kdump: Don't defer the reservation of crash high memory

[Baoquan He]

On 06/21/22 at 03:56pm, Leizhen (ThunderTown) wrote:
> 
> 
> On 2022/6/21 13:33, Baoquan He wrote:
> > Hi,
> > 
> > On 06/13/22 at 04:09pm, Zhen Lei wrote:
> >> If the crashkernel has both high memory above DMA zones and low memory
> >> in DMA zones, kexec always loads the content such as Image and dtb to the
> >> high memory instead of the low memory. This means that only high memory
> >> requires write protection based on page-level mapping. The allocation of
> >> high memory does not depend on the DMA boundary. So we can reserve the
> >> high memory first even if the crashkernel reservation is deferred.
> >>
> >> This means that the block mapping can still be performed on other kernel
> >> linear address spaces, the TLB miss rate can be reduced and the system
> >> performance will be improved.
> > 
> > Ugh, this looks a little ugly, honestly.
> > 
> > If that's for sure arm64 can't split large page mapping of linear
> > region, this patch is one way to optimize linear mapping. Given kdump
> > setting is necessary on arm64 server, the booting speed is truly
> > impacted heavily.
> 
> There is also a performance impact when running.

Yes, indeed, the TLB flush will happen more often.

> 
> > 
> > However, I would suggest letting it as is with below reasons:
> > 
> > 1) The code will complicate the crashkernel reservatoin code which
> > is already difficult to understand. 
> 
> Yeah, I feel it, too.
> 
> > 2) It can only optimize the two cases, first is CONFIG_ZONE_DMA|DMA32
> >   disabled, the other is crashkernel=,high is specified. While both
> >   two cases are corner case, most of systems have CONFIG_ZONE_DMA|DMA32
> >   enabled, and most of systems have crashkernel=xM which is enough.
> >   Having them optimized won't bring benefit to most of systems.
> 
> The case of CONFIG_ZONE_DMA|DMA32 disabled have been resolved by
> commit 031495635b46 ("arm64: Do not defer reserve_crashkernel() for platforms with no DMA memory zones").
> Currently the performance problem to be optimized is that DMA is enabled.

Yes, the disabled CONFIG_ZONE_DMA|DMA32 case has avoided the problem since
its boundary is decided already at that time. Crashkenrel=,high can slso
avoid this benefitting from the top done memblock allocating. However,
the crashkerne=xM which now gets the fallback support is the main syntax
we will use, that still has the problem.

> 
> 
> > 3) Besides, the crashkernel=,high can be handled earlier because 
> >   arm64 alwasys have memblock.bottom_up == false currently, thus we
> >   don't need worry arbout the lower limit of crashkernel,high
> >   reservation for now. If memblock.bottom_up is set true in the future,
> >   this patch doesn't work any more.
> > 
> > 
> > ...
> >         crash_base = memblock_phys_alloc_range(crash_size, CRASH_ALIGN,
> >                                                crash_base, crash_max);
> > 
> > So, in my opinion, we can leave the current NON_BLOCK|SECT mapping as
> > is caused by crashkernel reserving, since no regression is brought.
> > And meantime, turning to check if there's any way to make the contiguous
> > linear mapping and later splitting work. The patch 4, 5 in this patchset
> > doesn't make much sense to me, frankly speaking.
> 
> OK. As discussed earlier, I can rethink if there is a better way to patch 4-5,
> and this time focus on patch 1-2. In this way, all the functions are complete,
> and only optimization is left.

Sounds nice, thx.

> > 
> >>
> >> Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
> >> ---
> >>  arch/arm64/mm/init.c | 71 ++++++++++++++++++++++++++++++++++++++++----
> >>  1 file changed, 65 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> >> index fb24efbc46f5ef4..ae0bae2cafe6ab0 100644
> >> --- a/arch/arm64/mm/init.c
> >> +++ b/arch/arm64/mm/init.c
> >> @@ -141,15 +141,44 @@ static void __init reserve_crashkernel(int dma_state)
> >>  	unsigned long long crash_max = CRASH_ADDR_LOW_MAX;
> >>  	char *cmdline = boot_command_line;
> >>  	int dma_enabled = IS_ENABLED(CONFIG_ZONE_DMA) || IS_ENABLED(CONFIG_ZONE_DMA32);
> >> -	int ret;
> >> +	int ret, skip_res = 0, skip_low_res = 0;
> >>  	bool fixed_base;
> >>  
> >>  	if (!IS_ENABLED(CONFIG_KEXEC_CORE))
> >>  		return;
> >>  
> >> -	if ((!dma_enabled && (dma_state != DMA_PHYS_LIMIT_UNKNOWN)) ||
> >> -	     (dma_enabled && (dma_state != DMA_PHYS_LIMIT_KNOWN)))
> >> -		return;
> >> +	/*
> >> +	 * In the following table:
> >> +	 * X,high  means crashkernel=X,high
> >> +	 * unknown means dma_state = DMA_PHYS_LIMIT_UNKNOWN
> >> +	 * known   means dma_state = DMA_PHYS_LIMIT_KNOWN
> >> +	 *
> >> +	 * The first two columns indicate the status, and the last two
> >> +	 * columns indicate the phase in which crash high or low memory
> >> +	 * needs to be reserved.
> >> +	 *  ---------------------------------------------------
> >> +	 * | DMA enabled | X,high used |  unknown  |   known   |
> >> +	 *  ---------------------------------------------------
> >> +	 * |      N            N       |    low    |    NOP    |
> >> +	 * |      Y            N       |    NOP    |    low    |
> >> +	 * |      N            Y       |  high/low |    NOP    |
> >> +	 * |      Y            Y       |    high   |    low    |
> >> +	 *  ---------------------------------------------------
> >> +	 *
> >> +	 * But in this function, the crash high memory allocation of
> >> +	 * crashkernel=Y,high and the crash low memory allocation of
> >> +	 * crashkernel=X[@offset] for crashk_res are mixed at one place.
> >> +	 * So the table above need to be adjusted as below:
> >> +	 *  ---------------------------------------------------
> >> +	 * | DMA enabled | X,high used |  unknown  |   known   |
> >> +	 *  ---------------------------------------------------
> >> +	 * |      N            N       |    res    |    NOP    |
> >> +	 * |      Y            N       |    NOP    |    res    |
> >> +	 * |      N            Y       |res/low_res|    NOP    |
> >> +	 * |      Y            Y       |    res    |  low_res  |
> >> +	 *  ---------------------------------------------------
> >> +	 *
> >> +	 */
> >>  
> >>  	/* crashkernel=X[@offset] */
> >>  	ret = parse_crashkernel(cmdline, memblock_phys_mem_size(),
> >> @@ -169,10 +198,33 @@ static void __init reserve_crashkernel(int dma_state)
> >>  		else if (ret)
> >>  			return;
> >>  
> >> +		/* See the third row of the second table above, NOP */
> >> +		if (!dma_enabled && (dma_state == DMA_PHYS_LIMIT_KNOWN))
> >> +			return;
> >> +
> >> +		/* See the fourth row of the second table above */
> >> +		if (dma_enabled) {
> >> +			if (dma_state == DMA_PHYS_LIMIT_UNKNOWN)
> >> +				skip_low_res = 1;
> >> +			else
> >> +				skip_res = 1;
> >> +		}
> >> +
> >>  		crash_max = CRASH_ADDR_HIGH_MAX;
> >>  	} else if (ret || !crash_size) {
> >>  		/* The specified value is invalid */
> >>  		return;
> >> +	} else {
> >> +		/* See the 1-2 rows of the second table above, NOP */
> >> +		if ((!dma_enabled && (dma_state == DMA_PHYS_LIMIT_KNOWN)) ||
> >> +		     (dma_enabled && (dma_state == DMA_PHYS_LIMIT_UNKNOWN)))
> >> +			return;
> >> +	}
> >> +
> >> +	if (skip_res) {
> >> +		crash_base = crashk_res.start;
> >> +		crash_size = crashk_res.end - crashk_res.start + 1;
> >> +		goto check_low;
> >>  	}
> >>  
> >>  	fixed_base = !!crash_base;
> >> @@ -202,9 +254,18 @@ static void __init reserve_crashkernel(int dma_state)
> >>  		return;
> >>  	}
> >>  
> >> +	crashk_res.start = crash_base;
> >> +	crashk_res.end = crash_base + crash_size - 1;
> >> +
> >> +check_low:
> >> +	if (skip_low_res)
> >> +		return;
> >> +
> >>  	if ((crash_base >= CRASH_ADDR_LOW_MAX) &&
> >>  	     crash_low_size && reserve_crashkernel_low(crash_low_size)) {
> >>  		memblock_phys_free(crash_base, crash_size);
> >> +		crashk_res.start = 0;
> >> +		crashk_res.end = 0;
> >>  		return;
> >>  	}
> >>  
> >> @@ -219,8 +280,6 @@ static void __init reserve_crashkernel(int dma_state)
> >>  	if (crashk_low_res.end)
> >>  		kmemleak_ignore_phys(crashk_low_res.start);
> >>  
> >> -	crashk_res.start = crash_base;
> >> -	crashk_res.end = crash_base + crash_size - 1;
> >>  	insert_resource(&iomem_resource, &crashk_res);
> >>  }
> >>  
> >> -- 
> >> 2.25.1
> >>
> > 
> > .
> > 
> 
> -- 
> Regards,
>   Zhen Lei
>