Re: [PATCH 7/7] btrfs: accessors: factor out split memcpy with two sources

[Boris Burkov <boris@bur.io>]

On Tue, Jul 01, 2025 at 07:23:54PM +0200, David Sterba wrote:
> The case of a reading the bytes from 2 folios needs two memcpy()s, the
> compiler does not emit calls but two inline loops.
> 
> Factoring out the code makes some improvement (stack, code) and in the
> future will provide an optimized implementation as well. (The analogical
> version with two destinations is not done as it increases stack usage
> but can be done if needed.)

Is there some fundamental reason for this, or does it just happen to be
so? Sort of interesting either way. Does it make you worry that this
stuff will regress randomly in the future?

> 
> The address of the second folio is reordered before the first memcpy,
> which leads to an optimization reusing the vmemmap_base and
> page_offset_base (implementing folio_address()).
> 
> Stack usage reduction:
> 
>   btrfs_get_32                                           -8 (32 -> 24)
>   btrfs_get_64                                           -8 (32 -> 24)
> 
> Code size reduction:
> 
>      text    data     bss     dec     hex filename
>   1454279  115665   16088 1586032  183370 pre/btrfs.ko
>   1454229  115665   16088 1585982  18333e post/btrfs.ko
> 
>   DELTA: -50
> 
> As this is the last patch in this series, here's the overall diff
> starting and including commit "btrfs: accessors: simplify folio bounds
> checks":
> 
> Stack:
> 
>   btrfs_set_16                                          -72 (88 -> 16)
>   btrfs_get_32                                          -56 (80 -> 24)
>   btrfs_set_8                                           -72 (88 -> 16)
>   btrfs_set_64                                          -64 (88 -> 24)
>   btrfs_get_8                                           -72 (80 -> 8)
>   btrfs_get_16                                          -64 (80 -> 16)
>   btrfs_set_32                                          -64 (88 -> 24)
>   btrfs_get_64                                          -56 (80 -> 24)
> 
>   NEW (48):
> 	  report_setget_bounds                           48
>   LOST/NEW DELTA:      +48
>   PRE/POST DELTA:     -472
> 
> Code:
> 
>      text    data     bss     dec     hex filename
>   1456601  115665   16088 1588354  183c82 pre/btrfs.ko
>   1454229  115665   16088 1585982  18333e post/btrfs.ko
> 
>   DELTA: -2372

Sweet!

> 
> Signed-off-by: David Sterba <dsterba@suse.com>
> ---
>  fs/btrfs/accessors.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/btrfs/accessors.c b/fs/btrfs/accessors.c
> index af11f547371815..f554c4f723617f 100644
> --- a/fs/btrfs/accessors.c
> +++ b/fs/btrfs/accessors.c
> @@ -20,6 +20,15 @@ static void __cold report_setget_bounds(const struct extent_buffer *eb,
>  		   (unsigned long)ptr, eb->start, member_offset, size);
>  }
>  
> +/* Copy bytes from @src1 and @src2 to @dest. */
> +static __always_inline void memcpy_split_src(char *dest, const char *src1,
> +					     const char *src2, const size_t len1,
> +					     const size_t total)
> +{
> +	memcpy(dest, src1, len1);
> +	memcpy(dest + len1, src2, total - len1);
> +}
> +
>  /*
>   * Macro templates that define helpers to read/write extent buffer data of a
>   * given size, that are also used via ctree.h for access to item members by
> @@ -64,9 +73,9 @@ u##bits btrfs_get_##bits(const struct extent_buffer *eb,		\
>  		kaddr = folio_address(eb->folios[idx + 1]);		\
>  		lebytes[1] = *kaddr;					\
>  	} else {							\
> -		memcpy(lebytes, kaddr, part);				\
> -		kaddr = folio_address(eb->folios[idx + 1]);		\
> -		memcpy(lebytes + part, kaddr, sizeof(u##bits) - part);	\
> +		memcpy_split_src(lebytes, kaddr,			\
> +				 folio_address(eb->folios[idx + 1]),	\
> +				 part, sizeof(u##bits));		\
>  	}								\
>  	return get_unaligned_le##bits(lebytes);				\
>  }									\
> -- 
> 2.49.0
>