Re: [PATCH] NFS: Don't use page_file_mapping after removing the page

[Trond Myklebust <trondmy@hammerspace.com>]

On Mon, 2019-02-04 at 15:31 -0500, Benjamin Coddington wrote:
> On 4 Feb 2019, at 14:20, Benjamin Coddington wrote:
> 
> > On 4 Feb 2019, at 14:07, Benjamin Coddington wrote:
> > 
> > > On 4 Feb 2019, at 14:03, Benjamin Coddington wrote:
> > > 
> > > > On 4 Feb 2019, at 13:32, Trond Myklebust wrote:
> > > > 
> > > > > On Mon, 2019-02-04 at 13:13 -0500, Benjamin Coddington wrote:
> > > > > > If nfs_page_async_flush() removes the page from the
> > > > > > mapping, then we
> > > > > > can't use page_file_mapping() on it as nfs_updatepate() is
> > > > > > wont to do
> > > > > > when receiving an error.  Instead, simplify
> > > > > > nfs_zap_mapping() to take
> > > > > > the inode.
> > > > > > 
> > > > > 
> > > > > Won't this break NFS swap?
> > > > 
> > > > I guess it may, but then I wonder: is that a thing that anyone
> > > > does?  It
> > > > sounds like a terrible idea..
> > > > 
> > > > I'm curious enough though, and doing some research on it.
> > > 
> > > There's enough to commits in the tree fixing problem or
> > > supporting swap
> > > on NFS, so I suppose I've just missed all that.
> > 
> > So for diskless systems, ah.  It makes sense.
> > 
> > But does it make any sense to set NFS_INO_INVALID_DATA on a swap
> > file?
> > Does anyone share them?
> > 
> > Dazed and confused, but trying to continue..
> 
> So if the page is backing a swap file, nfs_zap_mapping() will fail to
> set
> NFS_INO_INVALID_DATA because i_mapping.nrpages will always be 0.
> 
> But it looks like NFS has already decided to not care if the mapping
> needs
> revalidation for swap, since we assume /shared/ swap files are
> insane.
> 
> So without testing swap on NFS at all, it seems safe.
> 

What about the case when the page is in the swap cache? Is that also
safe?

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com