Re: [PATCH v2] mm/memory_hotplug: Fix try_offline_node()

[David Hildenbrand]

On 01.11.19 23:11, David Hildenbrand wrote:
> try_offline_node() is pretty much broken right now:
> - The node span is updated when onlining memory, not when adding it. We
>    ignore memory that was mever onlined. Bad.
> - We touch possible garbage memmaps. The pfn_to_nid(pfn) can easily
>    trigger a kernel panic. Bad for memory that is offline but also bad
>    for subsection hotadd with ZONE_DEVICE, whereby the memmap of the first
>    PFN of a section might contain garbage.
> - Sections belonging to mixed nodes are not properly considered.
>
> As memory blocks might belong to multiple nodes, we would have to walk all
> pageblocks (or at least subsections) within present sections. However,
> we don't have a way to identify whether a memmap that is not online was
> initialized (relevant for ZONE_DEVICE). This makes things more complicated.
>
> Luckily, we can piggy pack on the node span and the nid stored in
> memory blocks. Currently, the node span is grown when calling
> move_pfn_range_to_zone() - e.g., when onlining memory, and shrunk when
> removing memory, before calling try_offline_node(). Sysfs links are
> created via link_mem_sections(), e.g., during boot or when adding memory.
>
> If the node still spans memory or if any memory block belongs to the
> nid, we don't set the node offline. As memory blocks that span multiple
> nodes cannot get offlined, the nid stored in memory blocks is reliable
> enough (for such online memory blocks, the node still spans the memory).
>
> Note: We will soon stop shrinking the ZONE_DEVICE zone and the node span
> when removing ZONE_DEVICE memory to fix similar issues (access of garbage
> memmaps) - until we have a reliable way to identify whether these memmaps
> were properly initialized. This implies later, that once a node had
> ZONE_DEVICE memory, we won't be able to set a node offline -
> which should be acceptable.
>
> Since commit f1dd2cd13c4b ("mm, memory_hotplug: do not associate hotadded
> memory to zones until online") memory that is added is not assoziated
> with a zone/node (memmap not initialized). The introducing
> commit 60a5a19e7419 ("memory-hotplug: remove sysfs file of node") already
> missed that we could have multiple nodes for a section and that the
> zone/node span is updated when onlining pages, not when adding them.
>
> I tested this by hotplugging two DIMMs to a memory-less and cpu-less NUMA
> node. The node is properly onlined when adding the DIMMs. When removing
> the DIMMs, the node is properly offlined.
>
> Fixes: 60a5a19e7419 ("memory-hotplug: remove sysfs file of node")
> Fixes: f1dd2cd13c4b ("mm, memory_hotplug: do not associate hotadded memory to zones until online") # visiable after d0dc12e86b319
> Cc: Tang Chen <tangchen@xxxxxxxxxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Keith Busch <keith.busch@xxxxxxxxx>
> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> Cc: "Peter Zijlstra (Intel)" <peterz@xxxxxxxxxxxxx>
> Cc: Jani Nikula <jani.nikula@xxxxxxxxx>
> Cc: Nayna Jain <nayna@xxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Oscar Salvador <osalvador@xxxxxxx>
> Cc: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>
> v1 -> v2:
> - Drop sysfs handling, simplify, and add a comment
> - Make sure to include last section fully
>
> We stop shrinking the ZONE_DEVICE zone after the following patch:
>   [PATCH v6 04/10] mm/memory_hotplug: Don't access uninitialized memmaps
>   in shrink_zone_span()
> This implies, the above note regarding ZONE_DEVICE on a node blocking a
> node from getting offlined until we sorted out how to properly shrink
> the ZONE_DEVICE zone.
>
> This patch is especially important for:
>   [PATCH v6 05/10] mm/memory_hotplug: Shrink zones when offlining
>   memory
> As the BUG fixed with this patch becomes now easier to observe when memory
> is offlined (in contrast to when memory would never have been onlined
> before).
>
> As both patches are stable fixes and in next/master for a long time, we
> should probably pull this patch in front of both and also backport this
> patch at least to
>   Cc: stable@xxxxxxxxxxxxxxx # v4.13+
> I have not checked yet if there are real blockers to do that. I guess not.
>
> ---
>   mm/memory_hotplug.c | 45 +++++++++++++++++++++++++++++----------------
>   1 file changed, 29 insertions(+), 16 deletions(-)
>
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 0140c20837b6..b5f696491577 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1634,6 +1634,18 @@ static int check_cpu_on_node(pg_data_t *pgdat)
>   	return 0;
>   }
>   
> +static int check_no_memblock_for_node_cb(struct memory_block *mem, void *arg)
> +{
> +	int nid = *(int *)arg;
> +
> +	/*
> +	 * If a memory block belongs to multiple nodes, the stored nid is not
> +	 * reliable. However, such blocks are always online (e.g., cannot get
> +	 * offlined) and, therefore, are still spanned by the node.
> +	 */
> +	return mem->nid == nid ? -EEXIST : 0;
> +}
> +
>   /**
>    * try_offline_node
>    * @nid: the node ID
> @@ -1645,26 +1657,27 @@ static int check_cpu_on_node(pg_data_t *pgdat)
>    */
>   void try_offline_node(int nid)
>   {
> +	const unsigned long end_section_nr = __highest_present_section_nr + 1;
>   	pg_data_t *pgdat = NODE_DATA(nid);
> -	unsigned long start_pfn = pgdat->node_start_pfn;
> -	unsigned long end_pfn = start_pfn + pgdat->node_spanned_pages;
> -	unsigned long pfn;
> -
> -	for (pfn = start_pfn; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
> -		unsigned long section_nr = pfn_to_section_nr(pfn);
> -
> -		if (!present_section_nr(section_nr))
> -			continue;
> +	int rc;
>   
> -		if (pfn_to_nid(pfn) != nid)
> -			continue;
> +	/*
> +	 * If the node still spans pages (especially ZONE_DEVICE), don't
> +	 * offline it. A node spans memory after move_pfn_range_to_zone(),
> +	 * e.g., after the memory block was onlined.
> +	 */
> +	if (pgdat->node_spanned_pages)
> +		return;
>   
> -		/*
> -		 * some memory sections of this node are not removed, and we
> -		 * can't offline node now.
> -		 */
> +	/*
> +	 * Especially offline memory blocks might not be spanned by the
> +	 * node. They will get spanned by the node once they get onlined.
> +	 * However, they link to the node in sysfs and can get onlined later.
> +	 */
> +	rc = walk_memory_blocks(0, PFN_PHYS(section_nr_to_pfn(end_section_nr)),
> +				&nid, check_no_memblock_for_node_cb);

walk_memory_block() might be fairly inefficient for this use case (as it 
uses subsys_find_device_by_id() on any possible memory block, which is a 
list scan).

I guess I will introduce a walk_each_memory_block() that uses 
bus_for_each_dev() under the hood.

Sorry for the noise :)


--

Thanks,

David / dhildenb