2021-10-05 09:35:19

by Geert Uytterhoeven

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

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: [email protected]
Signed-off-by: Geert Uytterhoeven <[email protected]>
---
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

2021-10-05 09:42:24

by Qu Wenruo

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

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: [email protected]
> Signed-off-by: Geert Uytterhoeven <[email protected]>

Oh, thanks for catching this.

Reviewed-by: Qu Wenruo <[email protected]>

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;
>
>

2021-10-05 23:28:13

by David Sterba

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

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: [email protected]
> Signed-off-by: Geert Uytterhoeven <[email protected]>

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