Date:   Fri, 28 Apr 2017 13:34:35 -0400 (EDT)
From:   Jerome Glisse <jglisse@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v2] mm, zone_device: replace {get,
 put}_zone_device_page() with a single reference

> Kirill points out that the calls to {get,put}_dev_pagemap() can be
> removed from the mm fast path if we take a single get_dev_pagemap()
> reference to signify that the page is alive and use the final put of the
> page to drop that reference.
> 
> This does require some care to make sure that any waits for the
> percpu_ref to drop to zero occur *after* devm_memremap_page_release(),
> since it now maintains its own elevated reference.

This is NAK from HMM point of view as i need those call. So if you remove
them now i will need to add them back as part of HMM.

Cheers,
Jérôme

> 
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Jérôme Glisse <jglisse@...hat.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Reviewed-by: Logan Gunthorpe <logang@...tatee.com>
> Suggested-by: Kirill Shutemov <kirill.shutemov@...ux.intel.com>
> Tested-by: Kirill Shutemov <kirill.shutemov@...ux.intel.com>
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
> ---
> Changes in v2:
> * Rebased to tip/master
> * Clarified comment in __put_page
> * Clarified devm_memremap_pages() kernel doc about ordering of
>   devm_memremap_pages_release() vs percpu_ref_kill() vs wait for
>   percpu_ref to drop to zero.
> 
> Ingo, I retested this with a revert of commit 6dd29b3df975 "Revert
> 'x86/mm/gup: Switch GUP to the generic get_user_page_fast()
> implementation'". It should be good to go through x86/mm.
> 
>  drivers/dax/pmem.c    |    2 +-
>  drivers/nvdimm/pmem.c |   13 +++++++++++--
>  include/linux/mm.h    |   14 --------------
>  kernel/memremap.c     |   22 +++++++++-------------
>  mm/swap.c             |   10 ++++++++++
>  5 files changed, 31 insertions(+), 30 deletions(-)
> 
> diff --git a/drivers/dax/pmem.c b/drivers/dax/pmem.c
> index 033f49b31fdc..cb0d742fa23f 100644
> --- a/drivers/dax/pmem.c
> +++ b/drivers/dax/pmem.c
> @@ -43,6 +43,7 @@ static void dax_pmem_percpu_exit(void *data)
>  	struct dax_pmem *dax_pmem = to_dax_pmem(ref);
>  
>  	dev_dbg(dax_pmem->dev, "%s\n", __func__);
> +	wait_for_completion(&dax_pmem->cmp);
>  	percpu_ref_exit(ref);
>  }
>  
> @@ -53,7 +54,6 @@ static void dax_pmem_percpu_kill(void *data)
>  
>  	dev_dbg(dax_pmem->dev, "%s\n", __func__);
>  	percpu_ref_kill(ref);
> -	wait_for_completion(&dax_pmem->cmp);
>  }
>  
>  static int dax_pmem_probe(struct device *dev)
> diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
> index 5b536be5a12e..fb7bbc79ac26 100644
> --- a/drivers/nvdimm/pmem.c
> +++ b/drivers/nvdimm/pmem.c
> @@ -25,6 +25,7 @@
>  #include <linux/badblocks.h>
>  #include <linux/memremap.h>
>  #include <linux/vmalloc.h>
> +#include <linux/blk-mq.h>
>  #include <linux/pfn_t.h>
>  #include <linux/slab.h>
>  #include <linux/pmem.h>
> @@ -231,6 +232,11 @@ static void pmem_release_queue(void *q)
>  	blk_cleanup_queue(q);
>  }
>  
> +static void pmem_freeze_queue(void *q)
> +{
> +	blk_mq_freeze_queue_start(q);
> +}
> +
>  static void pmem_release_disk(void *disk)
>  {
>  	del_gendisk(disk);
> @@ -284,6 +290,9 @@ static int pmem_attach_disk(struct device *dev,
>  	if (!q)
>  		return -ENOMEM;
>  
> +	if (devm_add_action_or_reset(dev, pmem_release_queue, q))
> +		return -ENOMEM;
> +
>  	pmem->pfn_flags = PFN_DEV;
>  	if (is_nd_pfn(dev)) {
>  		addr = devm_memremap_pages(dev, &pfn_res, &q->q_usage_counter,
> @@ -303,10 +312,10 @@ static int pmem_attach_disk(struct device *dev,
>  				pmem->size, ARCH_MEMREMAP_PMEM);
>  
>  	/*
> -	 * At release time the queue must be dead before
> +	 * At release time the queue must be frozen before
>  	 * devm_memremap_pages is unwound
>  	 */
> -	if (devm_add_action_or_reset(dev, pmem_release_queue, q))
> +	if (devm_add_action_or_reset(dev, pmem_freeze_queue, q))
>  		return -ENOMEM;
>  
>  	if (IS_ERR(addr))
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index a835edd2db34..695da2a19b4c 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -762,19 +762,11 @@ static inline enum zone_type page_zonenum(const struct
> page *page)
>  }
>  
>  #ifdef CONFIG_ZONE_DEVICE
> -void get_zone_device_page(struct page *page);
> -void put_zone_device_page(struct page *page);
>  static inline bool is_zone_device_page(const struct page *page)
>  {
>  	return page_zonenum(page) == ZONE_DEVICE;
>  }
>  #else
> -static inline void get_zone_device_page(struct page *page)
> -{
> -}
> -static inline void put_zone_device_page(struct page *page)
> -{
> -}
>  static inline bool is_zone_device_page(const struct page *page)
>  {
>  	return false;
> @@ -790,9 +782,6 @@ static inline void get_page(struct page *page)
>  	 */
>  	VM_BUG_ON_PAGE(page_ref_count(page) <= 0, page);
>  	page_ref_inc(page);
> -
> -	if (unlikely(is_zone_device_page(page)))
> -		get_zone_device_page(page);
>  }
>  
>  static inline void put_page(struct page *page)
> @@ -801,9 +790,6 @@ static inline void put_page(struct page *page)
>  
>  	if (put_page_testzero(page))
>  		__put_page(page);
> -
> -	if (unlikely(is_zone_device_page(page)))
> -		put_zone_device_page(page);
>  }
>  
>  #if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
> diff --git a/kernel/memremap.c b/kernel/memremap.c
> index 07e85e5229da..23a6483c3666 100644
> --- a/kernel/memremap.c
> +++ b/kernel/memremap.c
> @@ -182,18 +182,6 @@ struct page_map {
>  	struct vmem_altmap altmap;
>  };
>  
> -void get_zone_device_page(struct page *page)
> -{
> -	percpu_ref_get(page->pgmap->ref);
> -}
> -EXPORT_SYMBOL(get_zone_device_page);
> -
> -void put_zone_device_page(struct page *page)
> -{
> -	put_dev_pagemap(page->pgmap);
> -}
> -EXPORT_SYMBOL(put_zone_device_page);
> -
>  static void pgmap_radix_release(struct resource *res)
>  {
>  	resource_size_t key, align_start, align_size, align_end;
> @@ -237,6 +225,10 @@ static void devm_memremap_pages_release(struct device
> *dev, void *data)
>  	struct resource *res = &page_map->res;
>  	resource_size_t align_start, align_size;
>  	struct dev_pagemap *pgmap = &page_map->pgmap;
> +	unsigned long pfn;
> +
> +	for_each_device_pfn(pfn, page_map)
> +		put_page(pfn_to_page(pfn));
>  
>  	if (percpu_ref_tryget_live(pgmap->ref)) {
>  		dev_WARN(dev, "%s: page mapping is still live!\n", __func__);
> @@ -277,7 +269,10 @@ struct dev_pagemap *find_dev_pagemap(resource_size_t
> phys)
>   *
>   * Notes:
>   * 1/ @ref must be 'live' on entry and 'dead' before devm_memunmap_pages()
>   time
> - *    (or devm release event).
> + *    (or devm release event). The expected order of events is that @ref has
> + *    been through percpu_ref_kill() before devm_memremap_pages_release().
> The
> + *    wait for the completion of all references being dropped and
> + *    percpu_ref_exit() must occur after devm_memremap_pages_release().
>   *
>   * 2/ @res is expected to be a host memory range that could feasibly be
>   *    treated as a "System RAM" range, i.e. not a device mmio range, but
> @@ -379,6 +374,7 @@ void *devm_memremap_pages(struct device *dev, struct
> resource *res,
>  		 */
>  		list_del(&page->lru);
>  		page->pgmap = pgmap;
> +		percpu_ref_get(ref);
>  	}
>  	devres_add(dev, page_map);
>  	return __va(res->start);
> diff --git a/mm/swap.c b/mm/swap.c
> index 5dabf444d724..d8d9ee9e311a 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -97,6 +97,16 @@ static void __put_compound_page(struct page *page)
>  
>  void __put_page(struct page *page)
>  {
> +	if (is_zone_device_page(page)) {
> +		put_dev_pagemap(page->pgmap);
> +
> +		/*
> +		 * The page belongs to the device that created pgmap. Do
> +		 * not return it to page allocator.
> +		 */
> +		return;
> +	}
> +
>  	if (unlikely(PageCompound(page)))
>  		__put_compound_page(page);
>  	else
> 
> 

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