Re: [PATCH 05/10] btrfs: precalculate checksums per leaf once

[David Sterba <dsterba@suse.cz>]

On Mon, Nov 02, 2020 at 10:27:25PM +0800, Qu Wenruo wrote:
> On 2020/10/29 下午10:27, David Sterba wrote:
> > --- a/fs/btrfs/disk-io.c
> > +++ b/fs/btrfs/disk-io.c
> > @@ -3079,6 +3079,7 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
> >  	fs_info->sectorsize = sectorsize;
> >  	fs_info->sectorsize_bits = ilog2(sectorsize);
> >  	fs_info->csum_size = btrfs_super_csum_size(disk_super);
> > +	fs_info->csums_per_leaf = BTRFS_MAX_ITEM_SIZE(fs_info) / fs_info->csum_size;
> 
> I guess here we don't need any macro for division right?
> The BTRFS_MAX_ITEM_SIZE() should follow the type of
> BTRFS_MAX_ITEM_SIZE() which is u32, thus u32/u32, we're safe even on
> 32bit systems, right?

Yes, 32/32 is safe in general.

> >  	fs_info->stripesize = stripesize;
> >  
> >  	/*
> > diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> > index 29ac97248942..81440a0ba106 100644
> > --- a/fs/btrfs/extent-tree.c
> > +++ b/fs/btrfs/extent-tree.c
> > @@ -2138,17 +2138,10 @@ static u64 find_middle(struct rb_root *root)
> >   */
> >  u64 btrfs_csum_bytes_to_leaves(struct btrfs_fs_info *fs_info, u64 csum_bytes)
> >  {
> > -	u64 csum_size;
> > -	u64 num_csums_per_leaf;
> >  	u64 num_csums;
> >  
> > -	csum_size = BTRFS_MAX_ITEM_SIZE(fs_info);
> > -	num_csums_per_leaf = div64_u64(csum_size,
> > -			(u64)btrfs_super_csum_size(fs_info->super_copy));
> >  	num_csums = csum_bytes >> fs_info->sectorsize_bits;
> > -	num_csums += num_csums_per_leaf - 1;
> > -	num_csums = div64_u64(num_csums, num_csums_per_leaf);
> > -	return num_csums;
> > +	return DIV_ROUND_UP_ULL(num_csums, fs_info->csums_per_leaf);
> 
> Since it's just a DIV_ROUND_UP_ULL() call, can we make it inline?

Good point, but i'll check first if this does not bloat code due to
inlining. As it's called once in all callers, the overhead of call is
not a problem.