linux-kernel - Re: Odd NFS related SIGBUS (& possible fix)

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Date:	Tue, 19 Oct 2010 10:43:25 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Trond Myklebust <Trond.Myklebust@...app.com>
Cc:	Nick Piggin <npiggin@...nel.dk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...IV.linux.org.uk>,
	linux-fsdevel@...r.kernel.org
Subject: Re: Odd NFS related SIGBUS (& possible fix)

On Sat, 2010-10-02 at 06:57 +1000, Benjamin Herrenschmidt wrote:
> On Fri, 2010-10-01 at 14:35 -0400, Trond Myklebust wrote:
> > On Fri, 2010-10-01 at 14:12 -0400, Trond Myklebust wrote:
> > > However, it looks to me as if the right thing to do when the
> > > page->mapping has changed would be to do the same thing as
> > > block_page_mkwrite(), and just return VM_FAULT_NOPAGE so that the VM can
> > > retry the fault.
> > > IMO: We should only SIGBUS if the calls to nfs_flush_incompatible()
> > > and/or nfs_updatepage() fail.
> > > 
> > > Cheers
> > >   Trond
> > 
> > IOW: Something like the following patch.
> 
> Thanks, I'll try it but it will have to wait about 10 days as I'm off on
> vacation.
> 
> I'll see if somebody here can do some tests while I'm away

BTW. I got some reports from testers that the patch fixes the issue.

Thanks !

Cheers,
Ben.

> Cheers,
> Ben.
> 
> > Cheers
> >   Trond
> > 
> > ---------------------------------------------------------------------------
> > NFS: Don't SIGBUS if nfs_vm_page_mkwrite races with a cache invalidation
> > 
> > From: Trond Myklebust <Trond.Myklebust@...app.com>
> > 
> > In the case where we lock the page, and then find out that the page has
> > been thrown out of the page cache, we should just return VM_FAULT_NOPAGE.
> > This is what block_page_mkwrite() does in these situations.
> > 
> > Signed-off-by: Trond Myklebust <Trond.Myklebust@...app.com>
> > ---
> > 
> >  fs/nfs/file.c |   17 ++++++++---------
> >  1 files changed, 8 insertions(+), 9 deletions(-)
> > 
> > 
> > diff --git a/fs/nfs/file.c b/fs/nfs/file.c
> > index 05bf3c0..6d95e24 100644
> > --- a/fs/nfs/file.c
> > +++ b/fs/nfs/file.c
> > @@ -551,7 +551,7 @@ static int nfs_vm_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
> >  	struct file *filp = vma->vm_file;
> >  	struct dentry *dentry = filp->f_path.dentry;
> >  	unsigned pagelen;
> > -	int ret = -EINVAL;
> > +	int ret = VM_FAULT_NOPAGE;
> >  	struct address_space *mapping;
> >  
> >  	dfprintk(PAGECACHE, "NFS: vm_page_mkwrite(%s/%s(%ld), offset %lld)\n",
> > @@ -567,21 +567,20 @@ static int nfs_vm_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
> >  	if (mapping != dentry->d_inode->i_mapping)
> >  		goto out_unlock;
> >  
> > -	ret = 0;
> >  	pagelen = nfs_page_length(page);
> >  	if (pagelen == 0)
> >  		goto out_unlock;
> >  
> > -	ret = nfs_flush_incompatible(filp, page);
> > -	if (ret != 0)
> > -		goto out_unlock;
> > +	ret = VM_FAULT_LOCKED;
> > +	if (nfs_flush_incompatible(filp, page) == 0 &&
> > +	    nfs_updatepage(filp, page, 0, pagelen) == 0)
> > +		goto out;
> >  
> > -	ret = nfs_updatepage(filp, page, 0, pagelen);
> > +	ret = VM_FAULT_SIGBUS;
> >  out_unlock:
> > -	if (!ret)
> > -		return VM_FAULT_LOCKED;
> >  	unlock_page(page);
> > -	return VM_FAULT_SIGBUS;
> > +out:
> > +	return ret;
> >  }
> >  
> >  static const struct vm_operations_struct nfs_file_vm_ops = {
> > 
> > --
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> 
> 
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