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

[Yafang Shao]

On Fri, Dec 4, 2020 at 7:05 AM Matthew Wilcox <willy at infradead.org> wrote:
>
> Might be a good idea to cc Yafang on this ...
>

Thanks

> 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/
> >

I will post it soon.

> > 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



-- 
Thanks
Yafang