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Date:   Fri, 3 May 2019 12:06:19 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Pavel Tatashin <pasha.tatashin@...een.com>, jmorris@...ei.org,
        sashal@...nel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-nvdimm@...ts.01.org,
        akpm@...ux-foundation.org, mhocko@...e.com,
        dave.hansen@...ux.intel.com, dan.j.williams@...el.com,
        keith.busch@...el.com, vishal.l.verma@...el.com,
        dave.jiang@...el.com, zwisler@...nel.org, thomas.lendacky@....com,
        ying.huang@...el.com, fengguang.wu@...el.com, bp@...e.de,
        bhelgaas@...gle.com, baiyaowei@...s.chinamobile.com, tiwai@...e.de,
        jglisse@...hat.com
Subject: Re: [v5 2/3] mm/hotplug: make remove_memory() interface useable

On 02.05.19 20:43, Pavel Tatashin wrote:
> As of right now remove_memory() interface is inherently broken. It tries
> to remove memory but panics if some memory is not offline. The problem
> is that it is impossible to ensure that all memory blocks are offline as
> this function also takes lock_device_hotplug that is required to
> change memory state via sysfs.
> 

The existing interface can actually work today by registering a hotplug
notifier and rejecting any onlining attempts. But I agree that this way,
the interface becomes more usable.

> So, between calling this function and offlining all memory blocks there
> is always a window when lock_device_hotplug is released, and therefore,
> there is always a chance for a panic during this window.
> 
> Make this interface to return an error if memory removal fails. This way
> it is safe to call this function without panicking machine, and also
> makes it symmetric to add_memory() which already returns an error.
> 
> Signed-off-by: Pavel Tatashin <pasha.tatashin@...een.com>> ---
>  include/linux/memory_hotplug.h |  8 +++--
>  mm/memory_hotplug.c            | 61 ++++++++++++++++++++++------------
>  2 files changed, 46 insertions(+), 23 deletions(-)
> 
> diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
> index 8ade08c50d26..5438a2d92560 100644
> --- a/include/linux/memory_hotplug.h
> +++ b/include/linux/memory_hotplug.h
> @@ -304,7 +304,7 @@ static inline void pgdat_resize_init(struct pglist_data *pgdat) {}
>  extern bool is_mem_section_removable(unsigned long pfn, unsigned long nr_pages);
>  extern void try_offline_node(int nid);
>  extern int offline_pages(unsigned long start_pfn, unsigned long nr_pages);
> -extern void remove_memory(int nid, u64 start, u64 size);
> +extern int remove_memory(int nid, u64 start, u64 size);
>  extern void __remove_memory(int nid, u64 start, u64 size);
>  
>  #else
> @@ -321,7 +321,11 @@ static inline int offline_pages(unsigned long start_pfn, unsigned long nr_pages)
>  	return -EINVAL;
>  }
>  
> -static inline void remove_memory(int nid, u64 start, u64 size) {}
> +static inline bool remove_memory(int nid, u64 start, u64 size)
> +{
> +	return -EBUSY;
> +}
> +
>  static inline void __remove_memory(int nid, u64 start, u64 size) {}
>  #endif /* CONFIG_MEMORY_HOTREMOVE */
>  
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 8c454e82d4f6..a826aededa1a 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1778,9 +1778,10 @@ static int check_memblock_offlined_cb(struct memory_block *mem, void *arg)
>  		endpa = PFN_PHYS(section_nr_to_pfn(mem->end_section_nr + 1))-1;
>  		pr_warn("removing memory fails, because memory [%pa-%pa] is onlined\n",
>  			&beginpa, &endpa);
> -	}
>  
> -	return ret;
> +		return -EBUSY;
> +	}
> +	return 0;
>  }
>  
>  static int check_cpu_on_node(pg_data_t *pgdat)
> @@ -1843,19 +1844,9 @@ void try_offline_node(int nid)
>  }
>  EXPORT_SYMBOL(try_offline_node);
>  
> -/**
> - * remove_memory
> - * @nid: the node ID
> - * @start: physical address of the region to remove
> - * @size: size of the region to remove
> - *
> - * NOTE: The caller must call lock_device_hotplug() to serialize hotplug
> - * and online/offline operations before this call, as required by
> - * try_offline_node().
> - */
> -void __ref __remove_memory(int nid, u64 start, u64 size)
> +static int __ref try_remove_memory(int nid, u64 start, u64 size)
>  {
> -	int ret;
> +	int rc = 0;
>  
>  	BUG_ON(check_hotplug_memory_range(start, size));
>  
> @@ -1863,13 +1854,13 @@ void __ref __remove_memory(int nid, u64 start, u64 size)
>  
>  	/*
>  	 * All memory blocks must be offlined before removing memory.  Check
> -	 * whether all memory blocks in question are offline and trigger a BUG()
> +	 * whether all memory blocks in question are offline and return error
>  	 * if this is not the case.
>  	 */
> -	ret = walk_memory_range(PFN_DOWN(start), PFN_UP(start + size - 1), NULL,
> -				check_memblock_offlined_cb);
> -	if (ret)
> -		BUG();
> +	rc = walk_memory_range(PFN_DOWN(start), PFN_UP(start + size - 1), NULL,
> +			       check_memblock_offlined_cb);
> +	if (rc)
> +		goto done;
>  
>  	/* remove memmap entry */
>  	firmware_map_remove(start, start + size, "System RAM");
> @@ -1879,14 +1870,42 @@ void __ref __remove_memory(int nid, u64 start, u64 size)
>  
>  	try_offline_node(nid);
>  
> +done:
>  	mem_hotplug_done();
> +	return rc;
>  }
>  
> -void remove_memory(int nid, u64 start, u64 size)
> +/**
> + * remove_memory
> + * @nid: the node ID
> + * @start: physical address of the region to remove
> + * @size: size of the region to remove
> + *
> + * NOTE: The caller must call lock_device_hotplug() to serialize hotplug
> + * and online/offline operations before this call, as required by
> + * try_offline_node().
> + */
> +void __remove_memory(int nid, u64 start, u64 size)
>  {
> +
> +	/*
> +	 * trigger BUG() is some memory is not offlined prior to calling this
> +	 * function
> +	 */
> +	if (try_remove_memory(nid, start, size))
> +		BUG();
> +}
> +
> +/* Remove memory if every memory block is offline, otherwise return false */

Comment is wrong "return false"

> +int remove_memory(int nid, u64 start, u64 size)
> +{
> +	int rc;
> +
>  	lock_device_hotplug();
> -	__remove_memory(nid, start, size);
> +	rc  = try_remove_memory(nid, start, size);
>  	unlock_device_hotplug();
> +
> +	return rc;
>  }
>  EXPORT_SYMBOL_GPL(remove_memory);
>  #endif /* CONFIG_MEMORY_HOTREMOVE */
> 


Looks sane to me

Reviewed-by: David Hildenbrand <david@...hat.com>

-- 

Thanks,

David / dhildenb

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