Re: [PATCH] btrfs: allocate exact page array size in extent_buffer

[Chandan Rajendra <chandan@linux.vnet.ibm.com>]

On Thursday, July 14, 2016 02:29:32 PM David Sterba wrote:
> The calculation of extent_buffer::pages size was done for 4k PAGE_SIZE,
> but this wastes 15 unused pointers on arches with large page size. Eg.
> on ppc64 this gives 15 * 8 = 120 bytes.
>

The non PAGE_SIZE aligned extent buffer usage in page straddling tests in
test_eb_bitmaps() need atleast one more page. So how about the following ...

#define INLINE_EXTENT_BUFFER_PAGES    (BTRFS_MAX_METADATA_BLOCKSIZE / PAGE_SIZE + 1)


> Signed-off-by: David Sterba <dsterba@suse.com>
> ---
>  fs/btrfs/ctree.h     | 6 ------
>  fs/btrfs/extent_io.c | 2 ++
>  fs/btrfs/extent_io.h | 8 +++++++-
>  3 files changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index 4274a7bfdaed..f914f6187753 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -66,12 +66,6 @@ struct btrfs_ordered_sum;
>  #define BTRFS_COMPAT_EXTENT_TREE_V0
> 
>  /*
> - * the max metadata block size.  This limit is somewhat artificial,
> - * but the memmove costs go through the roof for larger blocks.
> - */
> -#define BTRFS_MAX_METADATA_BLOCKSIZE 65536
> -
> -/*
>   * we can actually store much bigger names, but lets not confuse the rest
>   * of linux
>   */
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index 75533adef998..6f468a1842e6 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -4660,6 +4660,8 @@ __alloc_extent_buffer(struct btrfs_fs_info *fs_info, u64 start,
>  	/*
>  	 * Sanity checks, currently the maximum is 64k covered by 16x 4k pages
>  	 */
> +	BUILD_BUG_ON(INLINE_EXTENT_BUFFER_PAGES * PAGE_SIZE
> +		!= BTRFS_MAX_METADATA_BLOCKSIZE);
>  	BUILD_BUG_ON(BTRFS_MAX_METADATA_BLOCKSIZE
>  		> MAX_INLINE_EXTENT_BUFFER_SIZE);
>  	BUG_ON(len > MAX_INLINE_EXTENT_BUFFER_SIZE);
> diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h
> index c0c1c4fef6ce..edfa1a0ab82b 100644
> --- a/fs/btrfs/extent_io.h
> +++ b/fs/btrfs/extent_io.h
> @@ -4,6 +4,12 @@
>  #include <linux/rbtree.h>
>  #include "ulist.h"
> 
> +/*
> + * The maximum metadata block size.  This limit is somewhat artificial,
> + * but the memmove costs go through the roof for larger blocks.
> + */
> +#define BTRFS_MAX_METADATA_BLOCKSIZE (65536U)
> +
>  /* bits for the extent state */
>  #define EXTENT_DIRTY		(1U << 0)
>  #define EXTENT_WRITEBACK	(1U << 1)
> @@ -118,7 +124,7 @@ struct extent_state {
>  #endif
>  };
> 
> -#define INLINE_EXTENT_BUFFER_PAGES 16
> +#define INLINE_EXTENT_BUFFER_PAGES    (BTRFS_MAX_METADATA_BLOCKSIZE / PAGE_SIZE)
>  #define MAX_INLINE_EXTENT_BUFFER_SIZE (INLINE_EXTENT_BUFFER_PAGES * PAGE_SIZE)
>  struct extent_buffer {
>  	u64 start;
> 

-- 
chandan