Re: [PATCH v3 23/25] mm/memremap_pages: Initialize all ZONE_DEVICE pages to start at refcount 0

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The DEVICE_PRIVATE/COHERENT changes look good to me so feel free to add:

Reviewed-by: Alistair Popple <apopple@xxxxxxxxxx>

Dan Williams <dan.j.williams@xxxxxxxxx> writes:

> The initial memremap_pages() implementation inherited the
> __init_single_page() default of pages starting life with an elevated
> reference count. This originally allowed for the page->pgmap pointer to
> alias with the storage for page->lru since a page was only allowed to be
> on an lru list when its reference count was zero.
>
> Since then, 'struct page' definition cleanups have arranged for
> dedicated space for the ZONE_DEVICE page metadata, the
> MEMORY_DEVICE_{PRIVATE,COHERENT} work has arranged for the 1 -> 0
> page->_refcount transition to route the page to free_zone_device_page()
> and not the core-mm page-free, and MEMORY_DEVICE_{PRIVATE,COHERENT} now
> arranges for its ZONE_DEVICE pages to start at _refcount 0. With those
> cleanups in place and with filesystem-dax and device-dax now converted
> to take and drop references at map and truncate time, it is possible to
> start MEMORY_DEVICE_FS_DAX and MEMORY_DEVICE_GENERIC reference counts at
> 0 as well.
>
> This conversion also unifies all @pgmap accounting to be relative to
> pgmap_request_folio() and the paired folio_put() calls for those
> requested folios. This allows pgmap_release_folios() to be simplified to
> just a folio_put() helper.
>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: "Darrick J. Wong" <djwong@xxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> Cc: Alistair Popple <apopple@xxxxxxxxxx>
> Cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
>  drivers/dax/mapping.c    |    2 +-
>  include/linux/dax.h      |    2 +-
>  include/linux/memremap.h |    6 ++----
>  mm/memremap.c            |   36 ++++++++++++++++--------------------
>  mm/page_alloc.c          |    9 +--------
>  5 files changed, 21 insertions(+), 34 deletions(-)
>
> diff --git a/drivers/dax/mapping.c b/drivers/dax/mapping.c
> index 07caaa23d476..ca06f2515644 100644
> --- a/drivers/dax/mapping.c
> +++ b/drivers/dax/mapping.c
> @@ -691,7 +691,7 @@ static struct page *dax_zap_pages(struct xa_state *xas, void *entry)
>
>  	dax_for_each_folio(entry, folio, i) {
>  		if (zap)
> -			pgmap_release_folios(folio_pgmap(folio), folio, 1);
> +			pgmap_release_folios(folio, 1);
>  		if (!ret && !dax_folio_idle(folio))
>  			ret = folio_page(folio, 0);
>  	}
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index f2fbb5746ffa..f4fc37933fc2 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -235,7 +235,7 @@ static inline void dax_unlock_mapping_entry(struct address_space *mapping,
>   */
>  static inline bool dax_page_idle(struct page *page)
>  {
> -	return page_ref_count(page) == 1;
> +	return page_ref_count(page) == 0;
>  }
>
>  static inline bool dax_folio_idle(struct folio *folio)
> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> index 3fb3809d71f3..ddb196ae0696 100644
> --- a/include/linux/memremap.h
> +++ b/include/linux/memremap.h
> @@ -195,8 +195,7 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
>  				    struct dev_pagemap *pgmap);
>  bool pgmap_request_folios(struct dev_pagemap *pgmap, struct folio *folio,
>  			  int nr_folios);
> -void pgmap_release_folios(struct dev_pagemap *pgmap, struct folio *folio,
> -			  int nr_folios);
> +void pgmap_release_folios(struct folio *folio, int nr_folios);
>  bool pgmap_pfn_valid(struct dev_pagemap *pgmap, unsigned long pfn);
>
>  unsigned long vmem_altmap_offset(struct vmem_altmap *altmap);
> @@ -238,8 +237,7 @@ static inline bool pgmap_request_folios(struct dev_pagemap *pgmap,
>  	return false;
>  }
>
> -static inline void pgmap_release_folios(struct dev_pagemap *pgmap,
> -					struct folio *folio, int nr_folios)
> +static inline void pgmap_release_folios(struct folio *folio, int nr_folios)
>  {
>  }
>
> diff --git a/mm/memremap.c b/mm/memremap.c
> index c46e700f5245..368ff41c560b 100644
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -469,8 +469,10 @@ EXPORT_SYMBOL_GPL(get_dev_pagemap);
>
>  void free_zone_device_page(struct page *page)
>  {
> -	if (WARN_ON_ONCE(!page->pgmap->ops || !page->pgmap->ops->page_free))
> -		return;
> +	struct dev_pagemap *pgmap = page->pgmap;
> +
> +	/* wake filesystem 'break dax layouts' waiters */
> +	wake_up_var(page);
>
>  	mem_cgroup_uncharge(page_folio(page));
>
> @@ -505,17 +507,9 @@ void free_zone_device_page(struct page *page)
>  	 * to clear page->mapping.
>  	 */
>  	page->mapping = NULL;
> -	page->pgmap->ops->page_free(page);
> -
> -	if (page->pgmap->type != MEMORY_DEVICE_PRIVATE &&
> -	    page->pgmap->type != MEMORY_DEVICE_COHERENT)
> -		/*
> -		 * Reset the page count to 1 to prepare for handing out the page
> -		 * again.
> -		 */
> -		set_page_count(page, 1);
> -	else
> -		put_dev_pagemap(page->pgmap);
> +	if (pgmap->ops && pgmap->ops->page_free)
> +		pgmap->ops->page_free(page);
> +	put_dev_pagemap(page->pgmap);
>  }
>
>  static bool folio_span_valid(struct dev_pagemap *pgmap, struct folio *folio,
> @@ -576,17 +570,19 @@ bool pgmap_request_folios(struct dev_pagemap *pgmap, struct folio *folio,
>  }
>  EXPORT_SYMBOL_GPL(pgmap_request_folios);
>
> -void pgmap_release_folios(struct dev_pagemap *pgmap, struct folio *folio, int nr_folios)
> +/*
> + * A symmetric helper to undo the page references acquired by
> + * pgmap_request_folios(), but the caller can also just arrange
> + * folio_put() on all the folios it acquired previously for the same
> + * effect.
> + */
> +void pgmap_release_folios(struct folio *folio, int nr_folios)
>  {
>  	struct folio *iter;
>  	int i;
>
> -	for (iter = folio, i = 0; i < nr_folios; iter = folio_next(iter), i++) {
> -		if (!put_devmap_managed_page(&iter->page))
> -			folio_put(iter);
> -		if (!folio_ref_count(iter))
> -			put_dev_pagemap(pgmap);
> -	}
> +	for (iter = folio, i = 0; i < nr_folios; iter = folio_next(folio), i++)
> +		folio_put(iter);
>  }
>
>  #ifdef CONFIG_FS_DAX
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 8e9b7f08a32c..e35d1eb3308d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6787,6 +6787,7 @@ static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
>  {
>
>  	__init_single_page(page, pfn, zone_idx, nid);
> +	set_page_count(page, 0);
>
>  	/*
>  	 * Mark page reserved as it will need to wait for onlining
> @@ -6819,14 +6820,6 @@ static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
>  		set_pageblock_migratetype(page, MIGRATE_MOVABLE);
>  		cond_resched();
>  	}
> -
> -	/*
> -	 * ZONE_DEVICE pages are released directly to the driver page allocator
> -	 * which will set the page count to 1 when allocating the page.
> -	 */
> -	if (pgmap->type == MEMORY_DEVICE_PRIVATE ||
> -	    pgmap->type == MEMORY_DEVICE_COHERENT)
> -		set_page_count(page, 0);
>  }
>
>  /*




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