Re: [PATCH] [RFC] Trigger retry from fault vm operation

[Matthew Wilcox]

On Tue, May 11, 2021 at 04:01:13PM +0200, Andreas Gruenbacher wrote:
> we have a locking problem in gfs2 that I don't have a proper solution for, so
> I'm looking for suggestions.
> 
> What's happening is that a page fault triggers during a read or write
> operation, while we're holding a glock (the cluster-wide gfs2 inode
> lock), and the page fault requires another glock.  We can recognize and
> handle the case when both glocks are the same, but when the page fault requires
> another glock, there is a chance that taking that other glock would deadlock.

So we're looking at something like one file on a gfs2 filesystem being
mmaped() and then doing read() or write() to another gfs2 file with the
mmaped address being the passed to read()/write()?

Have you looked at iov_iter_fault_in_readable() as a solution to
your locking order?  That way, you bring the mmaped page in first
(see generic_perform_write()).

> When we realize that we may not be able to take the other glock in gfs2_fault,
> we need to communicate that to the read or write operation, which will then
> drop and re-acquire the "outer" glock and retry.  However, there doesn't seem
> to be a good way to do that; we can only indicate that a page fault should fail
> by returning VM_FAULT_SIGBUS or similar; that will then be mapped to -EFAULT.
> We'd need something like VM_FAULT_RESTART that can be mapped to -EBUSY so that
> we can tell the retry case apart from genuine -EFAULT errors.

We do have VM_FAULT_RETRY ... does that retry at the wrong level?