Subject: Re: [PATCH v1 1/2] powerpc/pseries/hotplug-memory: stop checking is_mem_section_removable()

From: Baoquan He <[email-masked]>
Date: Wed, 8 Apr 2020 10:46:30 +0800

Add Pingfan to CC since he usually handles ppc related bugs for RHEL.

On 04/07/20 at 03:54pm, David Hildenbrand wrote:
> In commit 53cdc1cb29e8 ("drivers/base/memory.c: indicate all memory
> blocks as removable"), the user space interface to compute whether a memory
> block can be offlined (exposed via
> /sys/devices/system/memory/memoryX/removable) has effectively been
> deprecated. We want to remove the leftovers of the kernel implementation.

Pingfan, can you have a look at this change on PPC?  Please feel free to
give comments if any concern, or offer ack if it's OK to you.

> 
> When offlining a memory block (mm/memory_hotplug.c:__offline_pages()),
> we'll start by:
> 1. Testing if it contains any holes, and reject if so
> 2. Testing if pages belong to different zones, and reject if so
> 3. Isolating the page range, checking if it contains any unmovable pages
> 
> Using is_mem_section_removable() before trying to offline is not only racy,
> it can easily result in false positives/negatives. Let's stop manually
> checking is_mem_section_removable(), and let device_offline() handle it
> completely instead. We can remove the racy is_mem_section_removable()
> implementation next.
> 
> We now take more locks (e.g., memory hotplug lock when offlining and the
> zone lock when isolating), but maybe we should optimize that
> implementation instead if this ever becomes a real problem (after all,
> memory unplug is already an expensive operation). We started using
> is_mem_section_removable() in commit 51925fb3c5c9 ("powerpc/pseries:
> Implement memory hotplug remove in the kernel"), with the initial
> hotremove support of lmbs.
> 
> Cc: Nathan Fontenot <[email-masked]>
> Cc: Michael Ellerman <[email-masked]>
> Cc: Benjamin Herrenschmidt <[email-masked]>
> Cc: Paul Mackerras <[email-masked]>
> Cc: Michal Hocko <[email-masked]>
> Cc: Andrew Morton <[email-masked]>
> Cc: Oscar Salvador <[email-masked]>
> Cc: Baoquan He <[email-masked]>
> Cc: Wei Yang <[email-masked]>
> Signed-off-by: David Hildenbrand <[email-masked]>
> ---
>  .../platforms/pseries/hotplug-memory.c        | 26 +++----------------
>  1 file changed, 3 insertions(+), 23 deletions(-)
> 
> diff --git a/arch/powerpc/platforms/pseries/hotplug-memory.c b/arch/powerpc/platforms/pseries/hotplug-memory.c
> index b2cde1732301..5ace2f9a277e 100644
> --- a/arch/powerpc/platforms/pseries/hotplug-memory.c
> +++ b/arch/powerpc/platforms/pseries/hotplug-memory.c
> @@ -337,39 +337,19 @@ static int pseries_remove_mem_node(struct device_node *np)
>  
>  static bool lmb_is_removable(struct drmem_lmb *lmb)
>  {
> -	int i, scns_per_block;
> -	bool rc = true;
> -	unsigned long pfn, block_sz;
> -	u64 phys_addr;
> -
>  	if (!(lmb->flags & DRCONF_MEM_ASSIGNED))
>  		return false;
>  
> -	block_sz = memory_block_size_bytes();
> -	scns_per_block = block_sz / MIN_MEMORY_BLOCK_SIZE;
> -	phys_addr = lmb->base_addr;
> -
>  #ifdef CONFIG_FA_DUMP
>  	/*
>  	 * Don't hot-remove memory that falls in fadump boot memory area
>  	 * and memory that is reserved for capturing old kernel memory.
>  	 */
> -	if (is_fadump_memory_area(phys_addr, block_sz))
> +	if (is_fadump_memory_area(lmb->base_addr, memory_block_size_bytes()))
>  		return false;
>  #endif
> -
> -	for (i = 0; i < scns_per_block; i++) {
> -		pfn = PFN_DOWN(phys_addr);
> -		if (!pfn_in_present_section(pfn)) {
> -			phys_addr += MIN_MEMORY_BLOCK_SIZE;
> -			continue;
> -		}
> -
> -		rc = rc && is_mem_section_removable(pfn, PAGES_PER_SECTION);
> -		phys_addr += MIN_MEMORY_BLOCK_SIZE;
> -	}
> -
> -	return rc;
> +	/* device_offline() will determine if we can actually remove this lmb */
> +	return true;
>  }
>  
>  static int dlpar_add_lmb(struct drmem_lmb *);
> -- 
> 2.25.1
>