Re: [PATCH v3] mm/page_ext: Do not allocate space for page_ext->flags if not needed

[Vlastimil Babka]

On 1/13/23 16:42, Pasha Tatashin wrote:
> There is 8 byte page_ext->flags field allocated per page whenever
> CONFIG_PAGE_EXTENSION is enabled. However, not every user of page_ext
> uses flags. Therefore, check whether flags is needed at least by one
> user and if so allocate space for it.
> 
> For example when page_table_check is enabled, on a machine with 128G
> of memory before the fix:
> 
> [    2.244288] allocated 536870912 bytes of page_ext
> after the fix:
> [    2.160154] allocated 268435456 bytes of page_ext
> 
> Also, add a comment before page_ext_operations that describes the
> field, and remove check if need() is set, as that is now a required
> field.
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

Mike's comments look reasonable to me.

> ---
>  include/linux/page_ext.h | 19 +++++++++++++++++++
>  mm/page_ext.c            | 14 ++++++++++++--
>  mm/page_owner.c          |  1 +
>  mm/page_table_check.c    |  1 +
>  4 files changed, 33 insertions(+), 2 deletions(-)
> 
> Changelog:
> v2: Fixed field name in page_owner.c that caused build error.
> v3: Added comment before page_ext_operations, removed check if
>     need is null.
> 
> diff --git a/include/linux/page_ext.h b/include/linux/page_ext.h
> index 22be4582faae..be0b465c7d78 100644
> --- a/include/linux/page_ext.h
> +++ b/include/linux/page_ext.h
> @@ -7,15 +7,31 @@
>  #include <linux/stackdepot.h>
>  
>  struct pglist_data;
> +
> +/*
> + * Each Page Extension client must define page_ext_operations in
> + * page_ext_ops array.
> + * offset  offset to the client's data within page_ext. Offset is returned to
> + *         the client by page_ext_init.
> + * size    the size of the client data within page_ext.
> + * need()  function that returns true if client requires page_ext
> + * init()  (optional) called to initialize client once page_exts are allocated.
> + * using_shared_ext_flags  true when client is using shared page_ext->flags
> + *         field.
> + */
>  struct page_ext_operations {
>  	size_t offset;
>  	size_t size;
>  	bool (*need)(void);
>  	void (*init)(void);
> +	bool using_shared_ext_flags;
>  };
>  
>  #ifdef CONFIG_PAGE_EXTENSION
>  
> +/*
> + * The page_ext_flags users must set using_shared_ext_flags to true.
> + */
>  enum page_ext_flags {
>  	PAGE_EXT_OWNER,
>  	PAGE_EXT_OWNER_ALLOCATED,
> @@ -31,6 +47,9 @@ enum page_ext_flags {
>   * page_ext helps us add more information about the page.
>   * All page_ext are allocated at boot or memory hotplug event,
>   * then the page_ext for pfn always exists.
> + * Note: that if none of the users of page_ext is setting
> + * using_shared_ext_flags to true, the field cannot be used,
> + * as it will be overlapped with another data.
>   */
>  struct page_ext {
>  	unsigned long flags;
> diff --git a/mm/page_ext.c b/mm/page_ext.c
> index 4ee522fd381c..d14dbd44fa49 100644
> --- a/mm/page_ext.c
> +++ b/mm/page_ext.c
> @@ -71,6 +71,7 @@ static bool need_page_idle(void)
>  }
>  static struct page_ext_operations page_idle_ops __initdata = {
>  	.need = need_page_idle,
> +	.using_shared_ext_flags = true,
>  };
>  #endif
>  
> @@ -86,7 +87,7 @@ static struct page_ext_operations *page_ext_ops[] __initdata = {
>  #endif
>  };
>  
> -unsigned long page_ext_size = sizeof(struct page_ext);
> +unsigned long page_ext_size;
>  
>  static unsigned long total_usage;
>  static struct page_ext *lookup_page_ext(const struct page *page);
> @@ -106,7 +107,16 @@ static bool __init invoke_need_callbacks(void)
>  	bool need = false;
>  
>  	for (i = 0; i < entries; i++) {
> -		if (page_ext_ops[i]->need && page_ext_ops[i]->need()) {
> +		if (page_ext_ops[i]->need()) {
> +			if (page_ext_ops[i]->using_shared_ext_flags) {
> +				page_ext_size = sizeof(struct page_ext);
> +				break;
> +			}
> +		}
> +	}
> +
> +	for (i = 0; i < entries; i++) {
> +		if (page_ext_ops[i]->need()) {
>  			page_ext_ops[i]->offset = page_ext_size;
>  			page_ext_size += page_ext_ops[i]->size;
>  			need = true;
> diff --git a/mm/page_owner.c b/mm/page_owner.c
> index 2d27f532df4c..1a62ab3fc00e 100644
> --- a/mm/page_owner.c
> +++ b/mm/page_owner.c
> @@ -99,6 +99,7 @@ struct page_ext_operations page_owner_ops = {
>  	.size = sizeof(struct page_owner),
>  	.need = need_page_owner,
>  	.init = init_page_owner,
> +	.using_shared_ext_flags = true,
>  };
>  
>  static inline struct page_owner *get_page_owner(struct page_ext *page_ext)
> diff --git a/mm/page_table_check.c b/mm/page_table_check.c
> index 93e633c1d587..74cfe89d5f53 100644
> --- a/mm/page_table_check.c
> +++ b/mm/page_table_check.c
> @@ -45,6 +45,7 @@ struct page_ext_operations page_table_check_ops = {
>  	.size = sizeof(struct page_table_check),
>  	.need = need_page_table_check,
>  	.init = init_page_table_check,
> +	.using_shared_ext_flags = false,
>  };
>  
>  static struct page_table_check *get_page_table_check(struct page_ext *page_ext)