Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap

[Yi Zhang]

On 2018-09-25 at 13:21:24 -0700, Alexander Duyck wrote:
> The ZONE_DEVICE pages were being initialized in two locations. One was with
> the memory_hotplug lock held and another was outside of that lock. The
> problem with this is that it was nearly doubling the memory initialization
> time. Instead of doing this twice, once while holding a global lock and
> once without, I am opting to defer the initialization to the one outside of
> the lock. This allows us to avoid serializing the overhead for memory init
> and we can instead focus on per-node init times.
> 
> One issue I encountered is that devm_memremap_pages and
> hmm_devmmem_pages_create were initializing only the pgmap field the same
> way. One wasn't initializing hmm_data, and the other was initializing it to
> a poison value. Since this is something that is exposed to the driver in
> the case of hmm I am opting for a third option and just initializing
> hmm_data to 0 since this is going to be exposed to unknown third party
> drivers.
> 
> Reviewed-by: Pavel Tatashin <pavel.tatashin@xxxxxxxxxxxxx>
> Signed-off-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> ---
> 
> v4: Moved moved memmap_init_zone_device to below memmmap_init_zone to avoid
>     merge conflicts with other changes in the kernel.
> v5: No change
> 
>  include/linux/mm.h |    2 +
>  kernel/memremap.c  |   24 +++++---------
>  mm/hmm.c           |   12 ++++---
>  mm/page_alloc.c    |   92 ++++++++++++++++++++++++++++++++++++++++++++++++++--
>  4 files changed, 107 insertions(+), 23 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 06d7d7576f8d..7312fb78ef31 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -848,6 +848,8 @@ static inline bool is_zone_device_page(const struct page *page)
>  {
>  	return page_zonenum(page) == ZONE_DEVICE;
>  }
> +extern void memmap_init_zone_device(struct zone *, unsigned long,
> +				    unsigned long, struct dev_pagemap *);
>  #else
>  static inline bool is_zone_device_page(const struct page *page)
>  {
> diff --git a/kernel/memremap.c b/kernel/memremap.c
> index 5b8600d39931..d0c32e473f82 100644
> --- a/kernel/memremap.c
> +++ b/kernel/memremap.c
> @@ -175,10 +175,10 @@ void *devm_memremap_pages(struct device *dev, struct dev_pagemap *pgmap)
>  	struct vmem_altmap *altmap = pgmap->altmap_valid ?
>  			&pgmap->altmap : NULL;
>  	struct resource *res = &pgmap->res;
> -	unsigned long pfn, pgoff, order;
> +	struct dev_pagemap *conflict_pgmap;
>  	pgprot_t pgprot = PAGE_KERNEL;
> +	unsigned long pgoff, order;
>  	int error, nid, is_ram;
> -	struct dev_pagemap *conflict_pgmap;
>  
>  	align_start = res->start & ~(SECTION_SIZE - 1);
>  	align_size = ALIGN(res->start + resource_size(res), SECTION_SIZE)
> @@ -256,19 +256,13 @@ void *devm_memremap_pages(struct device *dev, struct dev_pagemap *pgmap)
>  	if (error)
>  		goto err_add_memory;
>  
> -	for_each_device_pfn(pfn, pgmap) {
> -		struct page *page = pfn_to_page(pfn);
> -
> -		/*
> -		 * ZONE_DEVICE pages union ->lru with a ->pgmap back
> -		 * pointer.  It is a bug if a ZONE_DEVICE page is ever
> -		 * freed or placed on a driver-private list.  Seed the
> -		 * storage with LIST_POISON* values.
> -		 */
> -		list_del(&page->lru);
> -		page->pgmap = pgmap;
> -		percpu_ref_get(pgmap->ref);
> -	}
> +	/*
> +	 * Initialization of the pages has been deferred until now in order
> +	 * to allow us to do the work while not holding the hotplug lock.
> +	 */
> +	memmap_init_zone_device(&NODE_DATA(nid)->node_zones[ZONE_DEVICE],
> +				align_start >> PAGE_SHIFT,
> +				align_size >> PAGE_SHIFT, pgmap);
>  
>  	devm_add_action(dev, devm_memremap_pages_release, pgmap);
>  
> diff --git a/mm/hmm.c b/mm/hmm.c
> index c968e49f7a0c..774d684fa2b4 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -1024,7 +1024,6 @@ static int hmm_devmem_pages_create(struct hmm_devmem *devmem)
>  	resource_size_t key, align_start, align_size, align_end;
>  	struct device *device = devmem->device;
>  	int ret, nid, is_ram;
> -	unsigned long pfn;
>  
>  	align_start = devmem->resource->start & ~(PA_SECTION_SIZE - 1);
>  	align_size = ALIGN(devmem->resource->start +
> @@ -1109,11 +1108,14 @@ static int hmm_devmem_pages_create(struct hmm_devmem *devmem)
>  				align_size >> PAGE_SHIFT, NULL);
>  	mem_hotplug_done();
>  
> -	for (pfn = devmem->pfn_first; pfn < devmem->pfn_last; pfn++) {
> -		struct page *page = pfn_to_page(pfn);
> +	/*
> +	 * Initialization of the pages has been deferred until now in order
> +	 * to allow us to do the work while not holding the hotplug lock.
> +	 */
> +	memmap_init_zone_device(&NODE_DATA(nid)->node_zones[ZONE_DEVICE],
> +				align_start >> PAGE_SHIFT,
> +				align_size >> PAGE_SHIFT, &devmem->pagemap);
>  
> -		page->pgmap = &devmem->pagemap;
> -	}
>  	return 0;
>  
>  error_add_memory:
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 926ad3083b28..7ec0997ded39 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5489,12 +5489,23 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
>  	if (highest_memmap_pfn < end_pfn - 1)
>  		highest_memmap_pfn = end_pfn - 1;
>  
> +#ifdef CONFIG_ZONE_DEVICE
>  	/*
>  	 * Honor reservation requested by the driver for this ZONE_DEVICE
> -	 * memory
> +	 * memory. We limit the total number of pages to initialize to just
> +	 * those that might contain the memory mapping. We will defer the
> +	 * ZONE_DEVICE page initialization until after we have released
> +	 * the hotplug lock.
>  	 */
> -	if (altmap && start_pfn == altmap->base_pfn)
> -		start_pfn += altmap->reserve;
> +	if (zone == ZONE_DEVICE) {
> +		if (!altmap)
> +			return;
> +
> +		if (start_pfn == altmap->base_pfn)
> +			start_pfn += altmap->reserve;
> +		end_pfn = altmap->base_pfn + vmem_altmap_offset(altmap);
> +	}
> +#endif
>  
>  	for (pfn = start_pfn; pfn < end_pfn; pfn++) {
>  		/*
> @@ -5538,6 +5549,81 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
>  	}
>  }
>  
> +#ifdef CONFIG_ZONE_DEVICE
> +void __ref memmap_init_zone_device(struct zone *zone,
> +				   unsigned long start_pfn,
> +				   unsigned long size,
> +				   struct dev_pagemap *pgmap)
> +{
> +	unsigned long pfn, end_pfn = start_pfn + size;
> +	struct pglist_data *pgdat = zone->zone_pgdat;
> +	unsigned long zone_idx = zone_idx(zone);
> +	unsigned long start = jiffies;
> +	int nid = pgdat->node_id;
> +
> +	if (WARN_ON_ONCE(!pgmap || !is_dev_zone(zone)))
> +		return;
> +
> +	/*
> +	 * The call to memmap_init_zone should have already taken care
> +	 * of the pages reserved for the memmap, so we can just jump to
> +	 * the end of that region and start processing the device pages.
> +	 */
> +	if (pgmap->altmap_valid) {
> +		struct vmem_altmap *altmap = &pgmap->altmap;
> +
> +		start_pfn = altmap->base_pfn + vmem_altmap_offset(altmap);
> +		size = end_pfn - start_pfn;
> +	}
> +
> +	for (pfn = start_pfn; pfn < end_pfn; pfn++) {
> +		struct page *page = pfn_to_page(pfn);
> +
> +		__init_single_page(page, pfn, zone_idx, nid);
> +
> +		/*
> +		 * Mark page reserved as it will need to wait for onlining
> +		 * phase for it to be fully associated with a zone.
> +		 *
> +		 * We can use the non-atomic __set_bit operation for setting
> +		 * the flag as we are still initializing the pages.
> +		 */
> +		__SetPageReserved(page);

So we need to hold the page reserved flag while memory onlining.
But after onlined, Do we neeed to clear the reserved flag in the DEV/FS
DAX memory type?
@Dan, What will going on with this?

Regards
Yi.

> +
> +		/*
> +		 * ZONE_DEVICE pages union ->lru with a ->pgmap back
> +		 * pointer and hmm_data.  It is a bug if a ZONE_DEVICE
> +		 * page is ever freed or placed on a driver-private list.
> +		 */
> +		page->pgmap = pgmap;
> +		page->hmm_data = 0;
> +
> +		/*
> +		 * Mark the block movable so that blocks are reserved for
> +		 * movable at startup. This will force kernel allocations
> +		 * to reserve their blocks rather than leaking throughout
> +		 * the address space during boot when many long-lived
> +		 * kernel allocations are made.
> +		 *
> +		 * bitmap is created for zone's valid pfn range. but memmap
> +		 * can be created for invalid pages (for alignment)
> +		 * check here not to call set_pageblock_migratetype() against
> +		 * pfn out of zone.
> +		 *
> +		 * Please note that MEMMAP_HOTPLUG path doesn't clear memmap
> +		 * because this is done early in sparse_add_one_section
> +		 */
> +		if (!(pfn & (pageblock_nr_pages - 1))) {
> +			set_pageblock_migratetype(page, MIGRATE_MOVABLE);
> +			cond_resched();
> +		}
> +	}
> +
> +	pr_info("%s initialised, %lu pages in %ums\n", dev_name(pgmap->dev),
> +		size, jiffies_to_msecs(jiffies - start));
> +}
> +
> +#endif
>  static void __meminit zone_init_free_lists(struct zone *zone)
>  {
>  	unsigned int order, t;
> 
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