On Tue, Jul 22, 2008 at 07:18:02PM -0600, Andreas Dilger wrote:
> On Jul 22, 2008 17:51 -0700, Mingming Cao wrote:
> > + * Calulate the total number of credits to reserve to fit
> > + * the modification of @num pages into a single transaction
> > + */
> > +int ext4_writepages_trans_blocks(struct inode *inode, int num)
> > +{
> > + int bpp = ext4_journal_blocks_per_page(inode);
> > + int nrblocks = num * bpp;
> > +
> > + if (!EXT4_I(inode)->i_flags & EXT4_EXTENTS_FL)
> > + return ext4_writepages_trans_blocks_old(inode, nrblocks);
>
> This should be "if (!(EXT4_I(inode)->i_flags & EXT_EXTENTS_FL))", and
> we should probably make it "unlikely()" since we expect most new files
> in an ext4 filesystem are extent mapped.
The cost of unlikely() can be pretty bad; the rule of thumb I've heard
is that unless it's less than 1% vs. 99%, you should probably avoid
using likely() and unlikely(). Given that there will be a fair number
of people who will be doing upgrades of existing ext3 filesystems, I
don't think using unlikely() would be a good choice here.
- Ted
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