[PATCH -next] btrfs: lzo: Mask instead of divide to fix 32-bit build

[PATCH -next] btrfs: lzo: Mask instead of divide to fix 32-bit build

[Geert Uytterhoeven]

On 32-bit builds (e.g. m68k):

    ERROR: modpost: "__umoddi3" [fs/btrfs/btrfs.ko] undefined!

"cur_in - start" is 64-bit, while sectorsize is u32.

As sectorsize is always a power of two, the 64-by-32 modulo operation
can be replaced by a much cheaper bitwise AND with "sectorsize - 1".

Fixes: 0078783c7089fc83 ("btrfs: rework lzo_compress_pages() to make it subpage compatible")
Reported-by: noreply@ellerman.id.au
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
---
Compile-tested only.
---
 fs/btrfs/lzo.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/btrfs/lzo.c b/fs/btrfs/lzo.c
index 47003cec4046f13e..d08f9eba49dab3cd 100644
--- a/fs/btrfs/lzo.c
+++ b/fs/btrfs/lzo.c
@@ -211,7 +211,7 @@ int lzo_compress_pages(struct list_head *ws, struct address_space *mapping,
 	 */
 	cur_out += LZO_LEN;
 	while (cur_in < start + len) {
-		u32 sector_off = (cur_in - start) % sectorsize;
+		u32 sector_off = (cur_in - start) & (sectorsize - 1);
 		u32 in_len;
 		size_t out_len;
 
-- 
2.25.1

Re: [PATCH -next] btrfs: lzo: Mask instead of divide to fix 32-bit build

[Qu Wenruo]

On 2021/10/5 17:34, Geert Uytterhoeven wrote:
> On 32-bit builds (e.g. m68k):
> 
>      ERROR: modpost: "__umoddi3" [fs/btrfs/btrfs.ko] undefined!
> 
> "cur_in - start" is 64-bit, while sectorsize is u32.
> 
> As sectorsize is always a power of two, the 64-by-32 modulo operation
> can be replaced by a much cheaper bitwise AND with "sectorsize - 1".
> 
> Fixes: 0078783c7089fc83 ("btrfs: rework lzo_compress_pages() to make it subpage compatible")
> Reported-by: noreply@ellerman.id.au
> Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

Oh, thanks for catching this.

Reviewed-by: Qu Wenruo <wqu@suse.com>

Thanks,
Qu
> ---
> Compile-tested only.
> ---
>   fs/btrfs/lzo.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/lzo.c b/fs/btrfs/lzo.c
> index 47003cec4046f13e..d08f9eba49dab3cd 100644
> --- a/fs/btrfs/lzo.c
> +++ b/fs/btrfs/lzo.c
> @@ -211,7 +211,7 @@ int lzo_compress_pages(struct list_head *ws, struct address_space *mapping,
>   	 */
>   	cur_out += LZO_LEN;
>   	while (cur_in < start + len) {
> -		u32 sector_off = (cur_in - start) % sectorsize;
> +		u32 sector_off = (cur_in - start) & (sectorsize - 1);
>   		u32 in_len;
>   		size_t out_len;
>   
> 

Re: [PATCH -next] btrfs: lzo: Mask instead of divide to fix 32-bit build

[David Sterba]

On Tue, Oct 05, 2021 at 11:34:06AM +0200, Geert Uytterhoeven wrote:
> On 32-bit builds (e.g. m68k):
> 
>     ERROR: modpost: "__umoddi3" [fs/btrfs/btrfs.ko] undefined!
> 
> "cur_in - start" is 64-bit, while sectorsize is u32.
> 
> As sectorsize is always a power of two, the 64-by-32 modulo operation
> can be replaced by a much cheaper bitwise AND with "sectorsize - 1".
> 
> Fixes: 0078783c7089fc83 ("btrfs: rework lzo_compress_pages() to make it subpage compatible")
> Reported-by: noreply@ellerman.id.au
> Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

Thanks. Folded to the patch as it's still in the devel queue.