Re: [PATCH v2] direct-io: Fix unsigned comparison overflow

[Eryu Guan <guaneryu@gmail.com>]

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.