Subject: Re: [PATCH v1 1/3] powerpc/mm/radix: Create separate mappings for hot-plugged memory

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Tue, 23 Jun 2020 16:01:02 +0530

Bharata B Rao <[email-masked]> writes:

> Memory that gets hot-plugged _during_ boot (and not the memory
> that gets plugged in after boot), is mapped with 1G mappings
> and will undergo splitting when it is unplugged. The splitting
> code has a few issues:
>
> 1. Recursive locking
> --------------------
> Memory unplug path takes cpu_hotplug_lock and calls stop_machine()
> for splitting the mappings. However stop_machine() takes
> cpu_hotplug_lock again causing deadlock.
>
> 2. BUG: sleeping function called from in_atomic() context
> ---------------------------------------------------------
> Memory unplug path (remove_pagetable) takes init_mm.page_table_lock
> spinlock and later calls stop_machine() which does wait_for_completion()
>
> 3. Bad unlock unbalance
> -----------------------
> Memory unplug path takes init_mm.page_table_lock spinlock and calls
> stop_machine(). The stop_machine thread function runs in a different
> thread context (migration thread) which tries to release and reaquire
> ptl. Releasing ptl from a different thread than which acquired it
> causes bad unlock unbalance.
>
> These problems can be avoided if we avoid mapping hot-plugged memory
> with 1G mapping, thereby removing the need for splitting them during
> unplug. Hence, during radix init, identify the hot-plugged memory region
> and create separate mappings for each LMB so that they don't get mapped
> with 1G mappings. The identification of hot-plugged memory has become
> possible after the commit b6eca183e23e ("powerpc/kernel: Enables memory
> hot-remove after reboot on pseries guests").
>
> To create separate mappings for every LMB in the hot-plugged
> region, we need lmb-size for which we use memory_block_size_bytes().
> Since this is early init time code, the machine type isn't probed yet
> and hence memory_block_size_bytes() would return the default LMB size
> as 16MB. Hence we end up issuing more number of mapping requests
> than earlier.

Considering we can split 1G pages correctly, we can avoid doing this?

>
> Signed-off-by: Bharata B Rao <[email-masked]>
> ---
>  arch/powerpc/mm/book3s64/radix_pgtable.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/mm/book3s64/radix_pgtable.c b/arch/powerpc/mm/book3s64/radix_pgtable.c
> index 8acb96de0e48..ffccfe00ca2a 100644
> --- a/arch/powerpc/mm/book3s64/radix_pgtable.c
> +++ b/arch/powerpc/mm/book3s64/radix_pgtable.c
> @@ -16,6 +16,7 @@
>  #include <linux/hugetlb.h>
>  #include <linux/string_helpers.h>
>  #include <linux/stop_machine.h>
> +#include <linux/memory.h>
>  
>  #include <asm/pgtable.h>
>  #include <asm/pgalloc.h>
> @@ -320,6 +321,8 @@ static void __init radix_init_pgtable(void)
>  {
>  	unsigned long rts_field;
>  	struct memblock_region *reg;
> +	phys_addr_t addr;
> +	u64 lmb_size = memory_block_size_bytes();
>  
>  	/* We don't support slb for radix */
>  	mmu_slb_size = 0;
> @@ -338,9 +341,15 @@ static void __init radix_init_pgtable(void)
>  			continue;
>  		}
>  
> -		WARN_ON(create_physical_mapping(reg->base,
> -						reg->base + reg->size,
> -						-1, PAGE_KERNEL));
> +		if (memblock_is_hotpluggable(reg)) {
> +			for (addr = reg->base; addr < (reg->base + reg->size);
> +			     addr += lmb_size)
> +				WARN_ON(create_physical_mapping(addr,
> +					addr + lmb_size, -1, PAGE_KERNEL));
> +		} else
> +			WARN_ON(create_physical_mapping(reg->base,
> +							reg->base + reg->size,
> +							-1, PAGE_KERNEL));
>  	}
>  
>  	/* Find out how many PID bits are supported */
> -- 
> 2.21.3