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Date:   Wed, 26 Sep 2018 09:55:40 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Alexander Duyck <alexander.h.duyck@linux.intel.com>
Cc:     linux-mm@kvack.org, akpm@linux-foundation.org,
        linux-kernel@vger.kernel.org, linux-nvdimm@lists.01.org,
        pavel.tatashin@microsoft.com, dave.jiang@intel.com,
        dave.hansen@intel.com, jglisse@redhat.com, rppt@linux.vnet.ibm.com,
        dan.j.williams@intel.com, logang@deltatee.com, mingo@kernel.org,
        kirill.shutemov@linux.intel.com
Subject: Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the
 point where we init pgmap
Message-ID: <20180926075540.GD6278@dhcp22.suse.cz>
References: <20180925200551.3576.18755.stgit@localhost.localdomain>
 <20180925202053.3576.66039.stgit@localhost.localdomain>
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On Tue 25-09-18 13:21:24, 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.

Why cannot you pull move_pfn_range_to_zone out of the hotplug lock? In
other words why are you making zone device even more special in the
generic hotplug code when it already has its own means to initialize the
pfn range by calling move_pfn_range_to_zone. Not to mention the code
duplication.

That being said I really dislike this patch.

> Reviewed-by: Pavel Tatashin <pavel.tatashin@microsoft.com>
> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> ---
> 
> 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);
> +
> +		/*
> +		 * 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;
> 

-- 
Michal Hocko
SUSE Labs