Re: [PATCH v4 02/11] fs: Allow fine-grained control of folio sizes

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, Apr 25, 2024 at 01:37:37PM +0200, Pankaj Raghav (Samsung) wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> 
> Some filesystems want to be able to ensure that folios that are added to
> the page cache are at least a certain size.
> Add mapping_set_folio_min_order() to allow this level of control.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Co-developed-by: Pankaj Raghav <p.raghav@samsung.com>
> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>

Looks good to me,
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
>  include/linux/pagemap.h | 116 +++++++++++++++++++++++++++++++++-------
>  1 file changed, 96 insertions(+), 20 deletions(-)
> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 2df35e65557d..2e5612de1749 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -202,13 +202,18 @@ enum mapping_flags {
>  	AS_EXITING	= 4, 	/* final truncate in progress */
>  	/* writeback related tags are not used */
>  	AS_NO_WRITEBACK_TAGS = 5,
> -	AS_LARGE_FOLIO_SUPPORT = 6,
> -	AS_RELEASE_ALWAYS,	/* Call ->release_folio(), even if no private data */
> -	AS_STABLE_WRITES,	/* must wait for writeback before modifying
> +	AS_RELEASE_ALWAYS = 6,	/* Call ->release_folio(), even if no private data */
> +	AS_STABLE_WRITES = 7,	/* must wait for writeback before modifying
>  				   folio contents */
> -	AS_UNMOVABLE,		/* The mapping cannot be moved, ever */
> +	AS_FOLIO_ORDER_MIN = 8,
> +	AS_FOLIO_ORDER_MAX = 13, /* Bit 8-17 are used for FOLIO_ORDER */
> +	AS_UNMOVABLE = 18,		/* The mapping cannot be moved, ever */
>  };
>  
> +#define AS_FOLIO_ORDER_MIN_MASK 0x00001f00
> +#define AS_FOLIO_ORDER_MAX_MASK 0x0003e000
> +#define AS_FOLIO_ORDER_MASK (AS_FOLIO_ORDER_MIN_MASK | AS_FOLIO_ORDER_MAX_MASK)
> +
>  /**
>   * mapping_set_error - record a writeback error in the address_space
>   * @mapping: the mapping in which an error should be set
> @@ -344,9 +349,63 @@ static inline void mapping_set_gfp_mask(struct address_space *m, gfp_t mask)
>  	m->gfp_mask = mask;
>  }
>  
> +/*
> + * There are some parts of the kernel which assume that PMD entries
> + * are exactly HPAGE_PMD_ORDER.  Those should be fixed, but until then,
> + * limit the maximum allocation order to PMD size.  I'm not aware of any
> + * assumptions about maximum order if THP are disabled, but 8 seems like
> + * a good order (that's 1MB if you're using 4kB pages)
> + */
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +#define MAX_PAGECACHE_ORDER	HPAGE_PMD_ORDER
> +#else
> +#define MAX_PAGECACHE_ORDER	8
> +#endif
> +
> +/*
> + * mapping_set_folio_order_range() - Set the folio order range
> + * @mapping: The address_space.
> + * @min: Minimum folio order (between 0-MAX_PAGECACHE_ORDER inclusive).
> + * @max: Maximum folio order (between @min-MAX_PAGECACHE_ORDER inclusive).
> + *
> + * The filesystem should call this function in its inode constructor to
> + * indicate which base size (min) and maximum size (max) of folio the VFS
> + * can use to cache the contents of the file.  This should only be used
> + * if the filesystem needs special handling of folio sizes (ie there is
> + * something the core cannot know).
> + * Do not tune it based on, eg, i_size.
> + *
> + * Context: This should not be called while the inode is active as it
> + * is non-atomic.
> + */
> +static inline void mapping_set_folio_order_range(struct address_space *mapping,
> +						 unsigned int min_order,
> +						 unsigned int max_order)
> +{
> +	if (min_order > MAX_PAGECACHE_ORDER)
> +		min_order = MAX_PAGECACHE_ORDER;
> +
> +	if (max_order > MAX_PAGECACHE_ORDER)
> +		max_order = MAX_PAGECACHE_ORDER;
> +
> +	max_order = max(max_order, min_order);
> +	/*
> +	 * TODO: max_order is not yet supported in filemap.
> +	 */
> +	mapping->flags = (mapping->flags & ~AS_FOLIO_ORDER_MASK) |
> +			 (min_order << AS_FOLIO_ORDER_MIN) |
> +			 (max_order << AS_FOLIO_ORDER_MAX);
> +}
> +
> +static inline void mapping_set_folio_min_order(struct address_space *mapping,
> +					       unsigned int min)
> +{
> +	mapping_set_folio_order_range(mapping, min, MAX_PAGECACHE_ORDER);
> +}
> +
>  /**
>   * mapping_set_large_folios() - Indicate the file supports large folios.
> - * @mapping: The file.
> + * @mapping: The address_space.
>   *
>   * The filesystem should call this function in its inode constructor to
>   * indicate that the VFS can use large folios to cache the contents of
> @@ -357,7 +416,37 @@ static inline void mapping_set_gfp_mask(struct address_space *m, gfp_t mask)
>   */
>  static inline void mapping_set_large_folios(struct address_space *mapping)
>  {
> -	__set_bit(AS_LARGE_FOLIO_SUPPORT, &mapping->flags);
> +	mapping_set_folio_order_range(mapping, 0, MAX_PAGECACHE_ORDER);
> +}
> +
> +static inline unsigned int mapping_max_folio_order(struct address_space *mapping)
> +{
> +	return (mapping->flags & AS_FOLIO_ORDER_MAX_MASK) >> AS_FOLIO_ORDER_MAX;
> +}
> +
> +static inline unsigned int mapping_min_folio_order(struct address_space *mapping)
> +{
> +	return (mapping->flags & AS_FOLIO_ORDER_MIN_MASK) >> AS_FOLIO_ORDER_MIN;
> +}
> +
> +static inline unsigned long mapping_min_folio_nrpages(struct address_space *mapping)
> +{
> +	return 1UL << mapping_min_folio_order(mapping);
> +}
> +
> +/**
> + * mapping_align_start_index() - Align starting index based on the min
> + * folio order of the page cache.
> + * @mapping: The address_space.
> + *
> + * Ensure the index used is aligned to the minimum folio order when adding
> + * new folios to the page cache by rounding down to the nearest minimum
> + * folio number of pages.
> + */
> +static inline pgoff_t mapping_align_start_index(struct address_space *mapping,
> +						pgoff_t index)
> +{
> +	return round_down(index, mapping_min_folio_nrpages(mapping));
>  }
>  
>  /*
> @@ -367,7 +456,7 @@ static inline void mapping_set_large_folios(struct address_space *mapping)
>  static inline bool mapping_large_folio_support(struct address_space *mapping)
>  {
>  	return IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
> -		test_bit(AS_LARGE_FOLIO_SUPPORT, &mapping->flags);
> +	       (mapping_max_folio_order(mapping) > 0);
>  }
>  
>  static inline int filemap_nr_thps(struct address_space *mapping)
> @@ -528,19 +617,6 @@ static inline void *detach_page_private(struct page *page)
>  	return folio_detach_private(page_folio(page));
>  }
>  
> -/*
> - * There are some parts of the kernel which assume that PMD entries
> - * are exactly HPAGE_PMD_ORDER.  Those should be fixed, but until then,
> - * limit the maximum allocation order to PMD size.  I'm not aware of any
> - * assumptions about maximum order if THP are disabled, but 8 seems like
> - * a good order (that's 1MB if you're using 4kB pages)
> - */
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -#define MAX_PAGECACHE_ORDER	HPAGE_PMD_ORDER
> -#else
> -#define MAX_PAGECACHE_ORDER	8
> -#endif
> -
>  #ifdef CONFIG_NUMA
>  struct folio *filemap_alloc_folio(gfp_t gfp, unsigned int order);
>  #else
> -- 
> 2.34.1
> 
>