Date:   Thu, 3 Feb 2022 11:22:41 +0900
From:   Jonghyeon Kim <tome01@...u.ac.kr>
To:     David Hildenbrand <david@...hat.com>
Cc:     dan.j.williams@...el.com, vishal.l.verma@...el.com,
        dave.jiang@...el.com, akpm@...ux-foundation.org,
        nvdimm@...ts.linux.dev, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 1/2] mm/memory_hotplug: Export shrink span functions for
 zone and node

On Fri, Jan 28, 2022 at 09:10:21AM +0100, David Hildenbrand wrote:
> On 28.01.22 05:19, Jonghyeon Kim wrote:
> > On Thu, Jan 27, 2022 at 10:54:23AM +0100, David Hildenbrand wrote:
> >> On 27.01.22 10:41, Jonghyeon Kim wrote:
> >>> On Wed, Jan 26, 2022 at 06:04:50PM +0100, David Hildenbrand wrote:
> >>>> On 26.01.22 18:00, Jonghyeon Kim wrote:
> >>>>> Export shrink_zone_span() and update_pgdat_span() functions to head
> >>>>> file. We need to update real number of spanned pages for NUMA nodes and
> >>>>> zones when we add memory device node such as device dax memory.
> >>>>>
> >>>>
> >>>> Can you elaborate a bit more what you intend to fix?
> >>>>
> >>>> Memory onlining/offlining is reponsible for updating the node/zone span,
> >>>> and that's triggered when the dax/kmem mamory gets onlined/offlined.
> >>>>
> >>> Sure, sorry for the lack of explanation of the intended fix.
> >>>
> >>> Before onlining nvdimm memory using dax(devdax or fsdax), these memory belong to
> >>> cpu NUMA nodes, which extends span pages of node/zone as a ZONE_DEVICE. So there
> >>> is no problem because node/zone contain these additional non-visible memory
> >>> devices to the system.
> >>> But, if we online dax-memory, zone[ZONE_DEVICE] of CPU NUMA node is hot-plugged
> >>> to new NUMA node(but CPU-less). I think there is no need to hold
> >>> zone[ZONE_DEVICE] pages on the original node.
> >>>
> >>> Additionally, spanned pages are also used to calculate the end pfn of a node.
> >>> Thus, it is needed to maintain accurate page stats for node/zone.
> >>>
> >>> My machine contains two CPU-socket consisting of DRAM and Intel DCPMM
> >>> (DC persistent memory modules) with App-Direct mode.
> >>>
> >>> Below are my test results.
> >>>
> >>> Before memory onlining:
> >>>
> >>> 	# ndctl create-namespace --mode=devdax
> >>> 	# ndctl create-namespace --mode=devdax
> >>> 	# cat /proc/zoneinfo | grep -E "Node|spanned" | paste - -
> >>> 	Node 0, zone      DMA	        spanned  4095
> >>> 	Node 0, zone    DMA32	        spanned  1044480
> >>> 	Node 0, zone   Normal	        spanned  7864320
> >>> 	Node 0, zone  Movable	        spanned  0
> >>> 	Node 0, zone   Device	        spanned  66060288
> >>> 	Node 1, zone      DMA	        spanned  0
> >>> 	Node 1, zone    DMA32	        spanned  0
> >>> 	Node 1, zone   Normal	        spanned  8388608
> >>> 	Node 1, zone  Movable	        spanned  0
> >>> 	Node 1, zone   Device	        spanned  66060288
> >>>
> >>> After memory onlining:
> >>>
> >>> 	# daxctl reconfigure-device --mode=system-ram --no-online dax0.0
> >>> 	# daxctl reconfigure-device --mode=system-ram --no-online dax1.0
> >>>
> >>> 	# cat /proc/zoneinfo | grep -E "Node|spanned" | paste - -
> >>> 	Node 0, zone      DMA	        spanned  4095
> >>> 	Node 0, zone    DMA32	        spanned  1044480
> >>> 	Node 0, zone   Normal	        spanned  7864320
> >>> 	Node 0, zone  Movable	        spanned  0
> >>> 	Node 0, zone   Device	        spanned  66060288
> >>> 	Node 1, zone      DMA	        spanned  0
> >>> 	Node 1, zone    DMA32	        spanned  0
> >>> 	Node 1, zone   Normal	        spanned  8388608
> >>> 	Node 1, zone  Movable	        spanned  0
> >>> 	Node 1, zone   Device	        spanned  66060288
> >>> 	Node 2, zone      DMA	        spanned  0
> >>> 	Node 2, zone    DMA32	        spanned  0
> >>> 	Node 2, zone   Normal	        spanned  65011712
> >>> 	Node 2, zone  Movable	        spanned  0
> >>> 	Node 2, zone   Device	        spanned  0
> >>> 	Node 3, zone      DMA	        spanned  0
> >>> 	Node 3, zone    DMA32	        spanned  0
> >>> 	Node 3, zone   Normal	        spanned  65011712
> >>> 	Node 3, zone  Movable	        spanned  0
> >>> 	Node 3, zone   Device	        spanned  0
> >>>
> >>> As we can see, Node 0 and 1 still have zone_device pages after memory onlining.
> >>> This causes problem that Node 0 and Node 2 have same end of pfn values, also 
> >>> Node 1 and Node 3 have same problem.
> >>
> >> Thanks for the information, that makes it clearer.
> >>
> >> While this unfortunate, the node/zone span is something fairly
> >> unreliable/unusable for user space. Nodes and zones can overlap just easily.
> >>
> >> What counts are present/managed pages in the node/zone.
> >>
> >> So at least I don't count this as something that "needs fixing",
> >> it's more something that's nice to handle better if easily possible.
> >>
> >> See below.
> >>
> >>>
> >>>>> Signed-off-by: Jonghyeon Kim <tome01@...u.ac.kr>
> >>>>> ---
> >>>>>  include/linux/memory_hotplug.h | 3 +++
> >>>>>  mm/memory_hotplug.c            | 6 ++++--
> >>>>>  2 files changed, 7 insertions(+), 2 deletions(-)
> >>>>>
> >>>>> diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
> >>>>> index be48e003a518..25c7f60c317e 100644
> >>>>> --- a/include/linux/memory_hotplug.h
> >>>>> +++ b/include/linux/memory_hotplug.h
> >>>>> @@ -337,6 +337,9 @@ extern void move_pfn_range_to_zone(struct zone *zone, unsigned long start_pfn,
> >>>>>  extern void remove_pfn_range_from_zone(struct zone *zone,
> >>>>>  				       unsigned long start_pfn,
> >>>>>  				       unsigned long nr_pages);
> >>>>> +extern void shrink_zone_span(struct zone *zone, unsigned long start_pfn,
> >>>>> +			     unsigned long end_pfn);
> >>>>> +extern void update_pgdat_span(struct pglist_data *pgdat);
> >>>>>  extern bool is_memblock_offlined(struct memory_block *mem);
> >>>>>  extern int sparse_add_section(int nid, unsigned long pfn,
> >>>>>  		unsigned long nr_pages, struct vmem_altmap *altmap);
> >>>>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> >>>>> index 2a9627dc784c..38f46a9ef853 100644
> >>>>> --- a/mm/memory_hotplug.c
> >>>>> +++ b/mm/memory_hotplug.c
> >>>>> @@ -389,7 +389,7 @@ static unsigned long find_biggest_section_pfn(int nid, struct zone *zone,
> >>>>>  	return 0;
> >>>>>  }
> >>>>>  
> >>>>> -static void shrink_zone_span(struct zone *zone, unsigned long start_pfn,
> >>>>> +void shrink_zone_span(struct zone *zone, unsigned long start_pfn,
> >>>>>  			     unsigned long end_pfn)
> >>>>>  {
> >>>>>  	unsigned long pfn;
> >>>>> @@ -428,8 +428,9 @@ static void shrink_zone_span(struct zone *zone, unsigned long start_pfn,
> >>>>>  		}
> >>>>>  	}
> >>>>>  }
> >>>>> +EXPORT_SYMBOL_GPL(shrink_zone_span);
> >>>>
> >>>> Exporting both as symbols feels very wrong. This is memory
> >>>> onlining/offlining internal stuff.
> >>>
> >>> I agree with you that your comment. I will find another approach to avoid
> >>> directly using onlining/offlining internal stuff while updating node/zone span.
> >>
> >> IIRC, to handle what you intend to handle properly want to look into teaching
> >> remove_pfn_range_from_zone() to handle zone_is_zone_device().
> >>
> >> There is a big fat comment:
> >>
> >> 	/*
> >> 	 * Zone shrinking code cannot properly deal with ZONE_DEVICE. So
> >> 	 * we will not try to shrink the zones - which is okay as
> >> 	 * set_zone_contiguous() cannot deal with ZONE_DEVICE either way.
> >> 	 */
> >> 	if (zone_is_zone_device(zone))
> >> 		return;
> >>
> >>
> >> Similarly, try_offline_node() spells this out:
> >>
> >> 	/*
> >> 	 * 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;
> >>
> >>
> >> So once you handle remove_pfn_range_from_zone() cleanly, you'll cleanly handle
> >> try_offline_node() implicitly.
> >>
> >> Trying to update the node span manually without teaching node/zone shrinking code how to
> >> handle ZONE_DEVICE properly is just a hack that will only sometimes work. Especially, it
> >> won't work if the range of interest is still surrounded by other ranges.
> >>
> > 
> > Thanks for your pointing out, I missed those comments.
> > I will keep trying to handle node/zone span updating process.
> 
> The only safe thing right now for on ZONE_DEVICE in
> remove_pfn_range_from_zone() would be removing the given range from the
> start/end of the zone range, but we must not scan using the existing
> functions.
> 
> As soon as we start actual *scanning* via find_smallest...
> find_biggest... in shrink_zone_span() we would mistakenly skip other
> ZONE_DEVICE ranges and mess up.
> 
> Assume you would have a ZONE_DEVICE layout like
> 
> [  DEV 0 | Hole | DEV 1 | Hole | DEV 2 ]
> 

IIUC, you assumed situation that multiple ZONE_DEVICE in single node, and there
are holes among them, right?

> What we actually want to do when removing
> 
> * DEV 0 is scanning low->high until we find DEV 1
> * DEV 1 is doing nothing, because we cannot shrink
> * DEV 2 is scanning high -> low until we find DEV 1
> 
> 
> I assume we'd want to call in shrink_zone_span() two new functions for
> ZONE_DEVICE:
> find_smallest_zone_device_pfn
> find_biggest_zone_device_pfn
> 
> Which would be able to do exactly that scanning, eventually, using
> get_dev_pagemap() or some similar source of information.
> 

I agree with your suggestion. It might cleanly deal with ZONE_DEVICE with actual
page scanning for checking holes.

> -- 
> Thanks,
> 
> David / dhildenb
> 

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