linux-kernel - Re: [PATCH -next] f2fs: Use div

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Date:   Fri, 21 Jun 2019 11:55:56 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Chao Yu <yuchao0@...wei.com>
Cc:     Qiuyang Sun <sunqiuyang@...wei.com>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        linux-f2fs-devel@...ts.sourceforge.net,
        Linux-Next <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] f2fs: Use div_u64*() for 64-bit divisions

Hi Chao,

On Fri, Jun 21, 2019 at 11:54 AM Chao Yu <yuchao0@...wei.com> wrote:
> Since the original patch hasn't been merged to upstream, I think we can merge
> this into original patch, how do you think?

Thanks, that's fine for me.

> On 2019/6/20 22:38, Geert Uytterhoeven wrote:
> > On 32-bit (e.g. m68k):
> >
> >     fs/f2fs/gc.o: In function `f2fs_resize_fs':
> >     gc.c:(.text+0x3056): undefined reference to `__umoddi3'
> >     gc.c:(.text+0x30c4): undefined reference to `__udivdi3'
> >
> > Fix this by using div_u64_rem() and div_u64() for 64-by-32 modulo resp.
> > division operations.
> >
> > Reported-by: noreply@...erman.id.au
> > Fixes: d2ae7494d043bfaf ("f2fs: ioctl for removing a range from F2FS")
> > Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>
> > ---
> > This assumes BLKS_PER_SEC(sbi) is 32-bit.
> >
> >     #define BLKS_PER_SEC(sbi)                                       \
> >           ((sbi)->segs_per_sec * (sbi)->blocks_per_seg)
> >
> > Notes:
> >   1. f2fs_sb_info.segs_per_sec and f2fs_sb_info.blocks_per_seg are both
> >      unsigned int,
> >   2. The multiplication is done in 32-bit arithmetic, hence the result
> >      is of type unsigned int.
> >   3. Is it guaranteed that the result will always fit in 32-bit, or can
> >      this overflow?
> >   4. fs/f2fs/debug.c:update_sit_info() assigns BLKS_PER_SEC(sbi) to
> >      unsigned long long blks_per_sec, anticipating a 64-bit value.
> > ---
> >  fs/f2fs/gc.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> > index 5b1076505ade9f84..c65f87f11de029f4 100644
> > --- a/fs/f2fs/gc.c
> > +++ b/fs/f2fs/gc.c
> > @@ -1438,13 +1438,15 @@ int f2fs_resize_fs(struct f2fs_sb_info *sbi, __u64 block_count)
> >       unsigned int secs;
> >       int gc_mode, gc_type;
> >       int err = 0;
> > +     __u32 rem;
> >
> >       old_block_count = le64_to_cpu(F2FS_RAW_SUPER(sbi)->block_count);
> >       if (block_count > old_block_count)
> >               return -EINVAL;
> >
> >       /* new fs size should align to section size */
> > -     if (block_count % BLKS_PER_SEC(sbi))
> > +     div_u64_rem(block_count, BLKS_PER_SEC(sbi), &rem);
> > +     if (rem)
> >               return -EINVAL;
> >
> >       if (block_count == old_block_count)
> > @@ -1463,7 +1465,7 @@ int f2fs_resize_fs(struct f2fs_sb_info *sbi, __u64 block_count)
> >       freeze_bdev(sbi->sb->s_bdev);
> >
> >       shrunk_blocks = old_block_count - block_count;
> > -     secs = shrunk_blocks / BLKS_PER_SEC(sbi);
> > +     secs = div_u64(shrunk_blocks, BLKS_PER_SEC(sbi));
> >       spin_lock(&sbi->stat_lock);
> >       if (shrunk_blocks + valid_user_blocks(sbi) +
> >               sbi->current_reserved_blocks + sbi->unusable_block_count +

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds