On Tue, Dec 05, 2017 at 02:19:27PM -0800, Matthew Wilcox wrote:
> On Tue, Dec 05, 2017 at 04:40:27PM -0500, Harish Kasiviswanathan wrote:
> > create = dio->op == REQ_OP_WRITE;
> > - if (dio->flags & DIO_SKIP_HOLES) {
> > + if (dio->flags & DIO_SKIP_HOLES &&
> > + i_size_read(dio->inode) > 0) {
> > if (fs_startblk <= ((i_size_read(dio->inode) - 1) >>
> > i_blkbits))
>
> i_size_read() isn't cheap on 32-bit SMP ... do we actually need to sample
> it at all here, or is it enough to use the i_size that was sampled earlier?
> IOW:
>
> create = dio->op == REQ_OP_WRITE;
> - if (dio->flags & DIO_SKIP_HOLES) {
> - if (fs_startblk <= ((i_size_read(dio->inode) - 1) >>
> - i_blkbits))
> + if (dio->flags & DIO_SKIP_HOLES && dio->i_size) {
> + if (fs_startblk <= (dio->i_size - 1) >> i_blkbits))
I think using dio->i_size should be fine. I tested ext3/4 with LTP
(aio-)dio tests and fstests and didn't see any regression introduced
with this change.
Thanks,
Eryu
>
> Another possibility would be to tweak the comparison slightly ...
>
> if (dio->flags & DIO_SKIP_HOLES) {
> - if (fs_startblk <= ((i_size_read(dio->inode) - 1) >>
> - i_blkbits))
> + if (fs_startblk < ((i_size_read(dio->inode) +
> + (1UL << i_blkbits) - 1) >> i_blkbits))
>
> Or we could use a temporary variable to avoid reading i_size twice.
