Re: [regression 4.2-rc3] loop: xfstests xfs/073 deadlocked in low memory conditions

Dave Chinner
2015-07-21T05:46:22Z

On Tue, Jul 21, 2015 at 12:05:56AM -0400, Ming Lei wrote:
> On Mon, Jul 20, 2015 at 9:59 PM, Dave Chinner <david@fromorbit.com> wrote:
> > Hi Ming,
> >
> > With the recent merge of the loop device changes, I'm now seeing
> > XFS deadlock on my single CPU, 1GB RAM VM running xfs/073.
> >
> > The deadlocked is as follows:
> >
> > kloopd1: loop_queue_read_work
> > xfs_file_iter_read
> > lock XFS inode XFS_IOLOCK_SHARED (on image file)
> > page cache read (GFP_KERNEL)
> > radix tree alloc
> > memory reclaim
> > reclaim XFS inodes
> > log force to unpin inodes
> > <wait for log IO completion>
> >
> > xfs-cil/loop1: <does log force IO work>
> > xlog_cil_push
> > xlog_write
> > <loop issuing log writes>
> > xlog_state_get_iclog_space()
> > <blocks due to all log buffers under write io>
> > <waits for IO completion>
> >
> > kloopd1: loop_queue_write_work
> > xfs_file_write_iter
> > lock XFS inode XFS_IOLOCK_EXCL (on image file)
> > <wait for inode to be unlocked>
> >
> > [The full stack traces are below].
> >
> > i.e. the kloopd, with it's split read and write work queues, has
> > introduced a dependency through memory reclaim. i.e. that writes
> > need to be able to progress for reads make progress.
>
> This kind of change just makes READ vs READ OR WRITE submitted
> to fs concurrently, and the use case should have been simulated from
> user space on one regular XFS file too?

Assuming the "regular XFS file" is on a normal block device (i.e.
not a loop device) then this will not deadlock as there is not
dependency on vfs level locking for log writes.

i.e. normal userspace IO path is:

userspace read
vfs_read
xfs_read
page cache alloc (GFP_KERNEL)
direct reclaim
xfs_inode reclaim
log force
CIL push
<workqueue>
xlog_write
submit_bio
-> hardware.

And then the log IO completes, and everything continues onward.

What the loop device used to do:

userspace read
vfs_read
xfs_read
page cache alloc (GFP_KERNEL)
submit_bio
loop device
splice_read (on image file)
xfs_splice_read
page cache alloc (GFP_NOFS)
direct reclaim
<skip filesystem reclaim>
submit_bio
-> hardware.

And when the read Io completes, everything moves onwards.

What the loop device now does:

userspace read
vfs_read
xfs_read
page cache alloc (GFP_KERNEL)
submit_bio
loop device
<workqueue>
vfs_read (on image file)
xfs_read
page cache alloc (GFP_KERNEL)
direct reclaim
xfs_inode reclaim
log force
CIL push
<workqueue>
xlog_write
submit_bio
loop device
<workqueue>
vfs_write (on image file)
xfs_write
<deadlock on image file lock>

> > The problem, fundamentally, is that mpage_readpages() does a
> > GFP_KERNEL allocation, rather than paying attention to the inode's
> > mapping gfp mask, which is set to GFP_NOFS.
>
> That looks the root cause, and I guess the issue is just triggered
> after commit aa4d86163e4(block: loop: switch to VFS ITER_BVEC)
> which changes splice to bvec iterator.

Yup - you are the unfortunate person who has wandered into the
minefield I'd been telling people about for quite some time. :(