On Mon, Jan 12, 2015 at 03:09:41PM -0800, Andrew Morton wrote:
> On Fri, 24 Oct 2014 17:20:38 -0400 Matthew Wilcox <matthew.r.wilcox@xxxxxxxxx> wrote:
> > +/*
> > + * When ext4 encounters a hole, it returns without modifying the buffer_head
> > + * which means that we can't trust b_size. To cope with this, we set b_state
> > + * to 0 before calling get_block and, if any bit is set, we know we can trust
> > + * b_size. Unfortunate, really, since ext4 knows precisely how long a hole is
> > + * and would save us time calling get_block repeatedly.
> > + */
> > +static bool buffer_size_valid(struct buffer_head *bh)
> > +{
> > + return bh->b_state != 0;
> > +}
>
> Yitch. Is there a cleaner way of doing this?
I'm hoping to fix ext* and then this problem can go away ...
> > +static ssize_t dax_io(int rw, struct inode *inode, struct iov_iter *iter,
> > + loff_t start, loff_t end, get_block_t get_block,
> > + struct buffer_head *bh)
>
> hm, some documentation would be nice. I expected "dax_io" to do IO,
> but this doesn't. Is it well named?
It does do I/O!
> > + if (rw == WRITE)
> > + len = copy_from_iter(addr, max - pos, iter);
> > + else if (!hole)
> > + len = copy_to_iter(addr, max - pos, iter);
> > + else
> > + len = iov_iter_zero(max - pos, iter);
> > + * This function uses the same locking scheme as do_blockdev_direct_IO:
> > + * If @flags has DIO_LOCKING set, we assume that the i_mutex is held by the
> > + * caller for writes. For reads, we take and release the i_mutex ourselves.
> > + * If DIO_LOCKING is not set, the filesystem takes care of its own locking.
> > + * As with do_blockdev_direct_IO(), we increment i_dio_count while the I/O
> > + * is in progress.
>
> It would be helpful here to explain *why* this code uses i_dio_count:
> what is trying to protect (against)?
Rather than just referencing the documentation in fs/direct_io.c? I
find it tends to get stale if we have documentation in multiple places.
> Oh, is that how it works ;)
>
> Perhaps a few BUG_ON(!mutex_is_locked(&inode->i_mutex)) would clarfiy
> and prevent mistakes.
Perhaps ... although there aren't any in blockdev_direct_IO(), and all the
callers are of the form:
if (IS_DAX)
dax_do_io()
else
blockdev_direct_IO()
so they've already got their flags and locking sorted out.
> > + */
> > +ssize_t dax_do_io(int rw, struct kiocb *iocb, struct inode *inode,
> > + struct iov_iter *iter, loff_t pos,
> > + get_block_t get_block, dio_iodone_t end_io, int flags)
> >
> > ...
> >
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