Re: [PATCH v6 1/7] arm64: mm: Move reserve_crashkernel() into mem_init()

[Nicolas Saenz Julienne]

On Thu, 2020-11-19 at 17:10 +0000, Catalin Marinas wrote:
> On Thu, Nov 19, 2020 at 03:09:58PM +0100, Nicolas Saenz Julienne wrote:
> > On Fri, 2020-11-13 at 11:29 +0000, Catalin Marinas wrote:
> > [...]
> > > > > > Let me stress that knowing the DMA constraints in the system before reserving
> > > > > > crashkernel's regions is necessary if we ever want it to work seamlessly on all
> > > > > > platforms. Be it small stuff like the Raspberry Pi or huge servers with TB of
> > > > > > memory.
> > > > > 
> > > > > Indeed. So we have 3 options (so far):
> > > > > 
> > > > > 1. Allow the crashkernel reservation to go into the linear map but set
> > > > >    it to invalid once allocated.
> > > > > 
> > > > > 2. Parse the flattened DT (not sure what we do with ACPI) before
> > > > >    creating the linear map. We may have to rely on some SoC ID here
> > > > >    instead of actual DMA ranges.
> > > > > 
> > > > > 3. Assume the smallest ZONE_DMA possible on arm64 (1GB) for crashkernel
> > > > >    reservations and not rely on arm64_dma_phys_limit in
> > > > >    reserve_crashkernel().
> > > > > 
> > > > > I think (2) we tried hard to avoid. Option (3) brings us back to the
> > > > > issues we had on large crashkernel reservations regressing on some
> > > > > platforms (though it's been a while since, they mostly went quiet ;)).
> > > > > However, with Chen's crashkernel patches we end up with two
> > > > > reservations, one in the low DMA zone and one higher, potentially above
> > > > > 4GB. Having a fixed 1GB limit wouldn't be any worse for crashkernel
> > > > > reservations than what we have now.
> > > > > 
> > > > > If (1) works, I'd go for it (James knows this part better than me),
> > > > > otherwise we can go for (3).
> > > > 
> > > > Overall, I'd prefer (1) as well, and I'd be happy to have a got at it. If not
> > > > I'll append (3) in this series.
> > > 
> > > I think for 1 we could also remove the additional KEXEC_CORE checks,
> > > something like below, untested:
> > > 
> > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > > index 3e5a6913acc8..27ab609c1c0c 100644
> > > --- a/arch/arm64/mm/mmu.c
> > > +++ b/arch/arm64/mm/mmu.c
> > > @@ -477,7 +477,8 @@ static void __init map_mem(pgd_t *pgdp)
> > >  	int flags = 0;
> > >  	u64 i;
> > >  
> > > -	if (rodata_full || debug_pagealloc_enabled())
> > > +	if (rodata_full || debug_pagealloc_enabled() ||
> > > +	    IS_ENABLED(CONFIG_KEXEC_CORE))
> > >  		flags = NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
> > >  
> > >  	/*
> > > @@ -487,11 +488,6 @@ static void __init map_mem(pgd_t *pgdp)
> > >  	 * the following for-loop
> > >  	 */
> > >  	memblock_mark_nomap(kernel_start, kernel_end - kernel_start);
> > > -#ifdef CONFIG_KEXEC_CORE
> > > -	if (crashk_res.end)
> > > -		memblock_mark_nomap(crashk_res.start,
> > > -				    resource_size(&crashk_res));
> > > -#endif
> > >  
> > >  	/* map all the memory banks */
> > >  	for_each_mem_range(i, &start, &end) {
> > > @@ -518,21 +514,6 @@ static void __init map_mem(pgd_t *pgdp)
> > >  	__map_memblock(pgdp, kernel_start, kernel_end,
> > >  		       PAGE_KERNEL, NO_CONT_MAPPINGS);
> > >  	memblock_clear_nomap(kernel_start, kernel_end - kernel_start);
> > > -
> > > -#ifdef CONFIG_KEXEC_CORE
> > > -	/*
> > > -	 * Use page-level mappings here so that we can shrink the region
> > > -	 * in page granularity and put back unused memory to buddy system
> > > -	 * through /sys/kernel/kexec_crash_size interface.
> > > -	 */
> > > -	if (crashk_res.end) {
> > > -		__map_memblock(pgdp, crashk_res.start, crashk_res.end + 1,
> > > -			       PAGE_KERNEL,
> > > -			       NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS);
> > > -		memblock_clear_nomap(crashk_res.start,
> > > -				     resource_size(&crashk_res));
> > > -	}
> > > -#endif
> > >  }
> > >  
> > >  void mark_rodata_ro(void)
> > 
> > So as far as I'm concerned this is good enough for me. I took the time to
> > properly test crashkernel on RPi4 using the series, this patch, and another
> > small fix to properly update /proc/iomem.
> > 
> > I'll send v7 soon, but before, James (or anyone for that matter) any obvious
> > push-back to Catalin's solution?
> 
> I talked to James earlier and he was suggesting that we check the
> command line for any crashkernel reservations and only disable block
> mappings in that case, see the diff below on top of the one I already
> sent (still testing it).

That's even better :)

> If you don't have any other changes for v7, I'm happy to pick v6 up on
> top of the no-block-mapping fix.

Yes I've got a small change in patch #1, the crashkernel reservation has to be
performed before request_standart_resouces() is called, which is OK, since
we're all setup by then, I moved the crashkernel reservation at the end of
bootmem_init(). I attached the patch. If it's easier for you I'll send v7.

Regards,
Nicolas

From 00dd2c31a027c42f80b76990a686000a36cc3bcf Mon Sep 17 00:00:00 2001
From: Nicolas Saenz Julienne <nsaenzjulienne@xxxxxxx>
Date: Wed, 14 Oct 2020 14:02:44 +0200
Subject: [PATCH] arm64: mm: Move reserve_crashkernel() into mem_init()

crashkernel might reserve memory located in ZONE_DMA. We plan to delay
ZONE_DMA's initialization after unflattening the devicetree and ACPI's
boot table initialization, so move it later in the boot process.
Specifically into bootmem_init() since request_standard_resources()
depends on it.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@xxxxxxx>
Tested-by: Jeremy Linton <jeremy.linton@xxxxxxx>

---

Changes since v6:
 - More reserve placement earlier.
---
 arch/arm64/mm/init.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 71d463544400..fafdf992fd32 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -389,8 +389,6 @@ void __init arm64_memblock_init(void)
 	else
 		arm64_dma32_phys_limit = PHYS_MASK + 1;
 
-	reserve_crashkernel();
-
 	reserve_elfcorehdr();
 
 	high_memory = __va(memblock_end_of_DRAM() - 1) + 1;
@@ -430,6 +428,12 @@ void __init bootmem_init(void)
 	sparse_init();
 	zone_sizes_init(min, max);
 
+	/*
+	 * request_standard_resources() depends on crashkernel's memory being
+	 * reserved, so do it here.
+	 */
+	reserve_crashkernel();
+
 	memblock_dump_all();
 }
 
-- 
2.29.2

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