[Linux-cachefs] Problems doing DIO to netfs cache on XFS from Ceph

[Matthew Wilcox]

Might be a good idea to cc Yafang on this ...

On Fri, Dec 04, 2020 at 09:12:02AM +1100, Dave Chinner wrote:
> On Thu, Dec 03, 2020 at 02:10:56PM +0000, David Howells wrote:
> > Hi Christoph,
> > 
> > We're having a problem making the fscache/cachefiles rewrite work with XFS, if
> > you could have a look?  Jeff Layton just tripped the attached warning from
> > this:
> > 
> > 	/*
> > 	 * Given that we do not allow direct reclaim to call us, we should
> > 	 * never be called in a recursive filesystem reclaim context.
> > 	 */
> > 	if (WARN_ON_ONCE(current->flags & PF_MEMALLOC_NOFS))
> > 		goto redirty;
> 
> I've pointed out in other threads where issues like this have been
> raised that this check is not correct and was broken some time ago
> by the PF_FSTRANS removal. The "NOFS" case here was originally using
> PF_FSTRANS to protect against recursion from within transaction
> contexts, not recursion through memory reclaim.  Doing writeback
> from memory reclaim is caught by the preceeding PF_MEMALLOC check,
> not this one.
> 
> What it is supposed to be warning about is that writeback in XFS can
> start new transactions and nesting transactions is a guaranteed way
> to deadlock the journal. IOWs, doing writeback from an active
> transaction context is a bug in XFS.
> 
> IOWs, we are waiting on a new version of this patchset to be posted:
> 
> https://lore.kernel.org/linux-xfs/20201103131754.94949-1-laoar.shao@gmail.com/
> 
> so that we can get rid of this from iomap and check the transaction
> recursion case directly in the XFS code. Then your problem goes away
> completely....
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david at fromorbit.com