[RFC PATCH v1 06/57] mm: Remove PAGE_SIZE compile-time constant assumption

[Vlastimil Babka]

On 10/14/24 12:58, Ryan Roberts wrote:
> To prepare for supporting boot-time page size selection, refactor code
> to remove assumptions about PAGE_SIZE being compile-time constant. Code
> intended to be equivalent when compile-time page size is active.
> 
> Refactor "struct vmap_block" to use a flexible array for used_mmap since
> VMAP_BBMAP_BITS is not a compile time constant for the boot-time page
> size case.
> 
> Update various BUILD_BUG_ON() instances to check against appropriate
> page size limit.
> 
> Re-define "union swap_header" so that it's no longer exactly page-sized.
> Instead define a flexible "magic" array with a define which tells the
> offset to where the magic signature begins.
> 
> Consider page size limit in some CPP condditionals.
> 
> Wrap global variables that are initialized with PAGE_SIZE derived values
> using DEFINE_GLOBAL_PAGE_SIZE_VAR() so their initialization can be
> deferred for boot-time page size builds.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts at arm.com>
> ---
> 
> ***NOTE***
> Any confused maintainers may want to read the cover note here for context:
> https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/
> 
>  drivers/mtd/mtdswap.c         |  4 ++--
>  include/linux/mm.h            |  2 +-
>  include/linux/mm_types_task.h |  2 +-
>  include/linux/mmzone.h        |  3 ++-
>  include/linux/slab.h          |  7 ++++---
>  include/linux/swap.h          | 17 ++++++++++++-----
>  include/linux/swapops.h       |  6 +++++-
>  mm/memcontrol.c               |  2 +-
>  mm/memory.c                   |  4 ++--
>  mm/mmap.c                     |  2 +-
>  mm/page-writeback.c           |  2 +-
>  mm/slub.c                     |  2 +-
>  mm/sparse.c                   |  2 +-
>  mm/swapfile.c                 |  2 +-
>  mm/vmalloc.c                  |  7 ++++---
>  15 files changed, 39 insertions(+), 25 deletions(-)
> 

> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -132,10 +132,17 @@ static inline int current_is_kswapd(void)
>   * bootbits...
>   */
>  union swap_header {
> -	struct {
> -		char reserved[PAGE_SIZE - 10];
> -		char magic[10];			/* SWAP-SPACE or SWAPSPACE2 */
> -	} magic;
> +	/*
> +	 * Exists conceptually, but since PAGE_SIZE may not be known at compile
> +	 * time, we must access through pointer arithmetic at run time.
> +	 *
> +	 * struct {
> +	 * 	char reserved[PAGE_SIZE - 10];
> +	 * 	char magic[10];			   SWAP-SPACE or SWAPSPACE2
> +	 * } magic;
> +	 */
> +#define SWAP_HEADER_MAGIC	(PAGE_SIZE - 10)
> +	char magic[1];

I wonder if it makes sense to even keep this magic field anymore.

>  	struct {
>  		char		bootbits[1024];	/* Space for disklabel etc. */
>  		__u32		version;
> @@ -201,7 +208,7 @@ struct swap_extent {
>   * Max bad pages in the new format..
>   */
>  #define MAX_SWAP_BADPAGES \
> -	((offsetof(union swap_header, magic.magic) - \
> +	((SWAP_HEADER_MAGIC - \
>  	  offsetof(union swap_header, info.badpages)) / sizeof(int))
>  
>  enum {

<snip>

> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -2931,7 +2931,7 @@ static unsigned long read_swap_header(struct swap_info_struct *p,
>  	unsigned long swapfilepages;
>  	unsigned long last_page;
>  
> -	if (memcmp("SWAPSPACE2", swap_header->magic.magic, 10)) {
> +	if (memcmp("SWAPSPACE2", &swap_header->magic[SWAP_HEADER_MAGIC], 10)) {

I'd expect static checkers to scream here because we overflow the magic[1]
both due to copying 10 bytes into 1 byte array and also with the insane
offset. Hence my suggestion to drop the field and use purely pointer arithmetic.

>  		pr_err("Unable to find swap-space signature\n");
>  		return 0;
>  	}
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index a0df1e2e155a8..b4fbba204603c 100644

Hm I'm actually looking at yourwip branch which also has:

--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -969,7 +969,7 @@ static inline int get_order_from_str(const char *size_str)
        return -EINVAL;
 }

-static char str_dup[PAGE_SIZE] __initdata;
+static char str_dup[PAGE_SIZE_MAX] __initdata;
 static int __init setup_thp_anon(char *str)
 {
        char *token, *range, *policy, *subtoken;

Why PAGE_SIZE_MAX? Isn't this the same case as "mm/memcontrol: Fix seq_buf
size to save memory when PAGE_SIZE is large"