Re: [PATCH v2 5/5] dax: handle media errors in dax_do_io

Jeff Moyer
2016-04-15T17:11:24Z

"Verma, Vishal L" <vishal.l.verma@intel.com> writes:

> On Fri, 2016-04-15 at 12:11 -0400, Jeff Moyer wrote:
>> Vishal Verma <vishal.l.verma@intel.com> writes:
>> > + if (IS_DAX(inode)) {
>> > + ret = dax_do_io(iocb, inode, iter, offset,
>> > blkdev_get_block,
>> >   NULL, DIO_SKIP_DIO_COUNT);
>> > - return __blockdev_direct_IO(iocb, inode, I_BDEV(inode),
>> > iter, offset,
>> > + if (ret == -EIO && (iov_iter_rw(iter) == WRITE))
>> > + ret_saved = ret;
>> > + else
>> > + return ret;
>> > + }
>> > +
>> > + ret = __blockdev_direct_IO(iocb, inode, I_BDEV(inode),
>> > iter, offset,
>> >       blkdev_get_block, NULL, NULL,
>> >       DIO_SKIP_DIO_COUNT);
>> > + if (ret < 0 && ret_saved)
>> > + return ret_saved;
>> > +
>> Hmm, did you just break async DIO?  I think you did!  :)
>> __blockdev_direct_IO can return -EIOCBQUEUED, and you've now turned
>> that
>> into -EIO.  Really, I don't see a reason to save that first
>> -EIO.  The
>> same applies to all instances in this patch.
>
> The reason I saved it was if __blockdev_direct_IO fails for some
> reason, we should return the original cause o the error, which was an
> EIO.. i.e. we shouldn't be hiding the EIO if the direct_IO fails with
> something else..

OK.

> But, how does _EIOCBQUEUED work? Maybe we need an exception for it?

For async direct I/O, only the setup phase of the I/O is performed and
then we return to the caller. -EIOCBQUEUED signifies this.

You're heading towards code that looks like this:

if (IS_DAX(inode)) {
ret = dax_do_io(iocb, inode, iter, offset, blkdev_get_block,
NULL, DIO_SKIP_DIO_COUNT);
if (ret == -EIO && (iov_iter_rw(iter) == WRITE))
ret_saved = ret;
else
return ret;
}

ret = __blockdev_direct_IO(iocb, inode, I_BDEV(inode), iter, offset,
blkdev_get_block, NULL, NULL,
DIO_SKIP_DIO_COUNT);
if (ret < 0 && ret != -EIOCBQUEUED && ret_saved)
return ret_saved;

There's a lot of special casing here, so you might consider adding
comments.

Cheers,
Jeff