Re: ext4 dio RWF_NOWAIT change

[Murphy Zhou <jencce.kernel@gmail.com>]

On Fri, Feb 07, 2020 at 10:38:24AM -0500, Theodore Y. Ts'o wrote:
> On Wed, Feb 05, 2020 at 05:13:44PM +0800, Murphy Zhou wrote:
> > Hi,
> > 
> > Kernel commit 378f32bab3714f04c4e0c3aee4129f6703805550
> > Author: Matthew Bobrowski <mbobrowski@mbobrowski.org>
> > Date:   Tue Nov 5 23:02:39 2019 +1100
> > 
> >     ext4: introduce direct I/O write using iomap infrastructure
> > 
> > 
> > Changed the logic of dio+RWF_NOWAIT
> > 
> > from:
> > 
> > -       if (!inode_trylock(inode)) {
> > -               if (iocb->ki_flags & IOCB_NOWAIT)
> > -                       return -EAGAIN;
> > -               inode_lock(inode);
> > -       }
> > 
> > 
> > to:
> > 
> > +       if (iocb->ki_flags & IOCB_NOWAIT) {
> > +               if (!inode_trylock(inode))
> > +                       return -EAGAIN;
> > +       } else {
> > +               inode_lock(inode);
> > +       }
> > 
> > 
> > fstests generic/471 expecet EAGAIN on this situation, so it started to
> > fail since than.
> 
> I don't understand why this specific change would cause the situation.
> In the generic/471 test, here iocb->ki_flags will have IOCB_NOWAIT
> set, and in that case I don't see how there would be a change in
> behavior with respect to EAGAIN being returned.
> 
> In any case, I've been suppressing generic/471 because of concerns
> that the test is bogus so I hadn't noticed.  From
> kvm-xfstests/test-appliance/files/root/fs/global_exclude:
> 
> # The test generic/471 tests the RWF_NOWAIT flag; however
> # how it is supposed to work with file systems is disputed,
> # and not all device drivers support it.  As a result
> # it doesn't work if an LVM device driver is in use (as is the
> # case with gce-xfstests).  So let's suppress it for now.  For
> # more details see:
> # https://lore.kernel.org/linux-block/20190723220502.GX7777@dread.disaster.area/
> generic/471
> 
> 							- Ted

Thanks for all the details! Ted.

Murphy