Re: [PATCH 3/3] fs: Fix remaining filesystems to wait for stable page writeback

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Nov 01, 2012 at 04:22:54PM -0400, Jeff Layton wrote:
> On Thu, 1 Nov 2012 11:43:26 -0700
> Boaz Harrosh <bharrosh@panasas.com> wrote:
> 
> > On 11/01/2012 12:58 AM, Darrick J. Wong wrote:
> > > Fix up the filesystems that provide their own ->page_mkwrite handlers to
> > > provide stable page writes if necessary.
> > > 
> > > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > > ---
> > >  fs/9p/vfs_file.c |    1 +
> > >  fs/afs/write.c   |    4 ++--
> > >  fs/ceph/addr.c   |    1 +
> > >  fs/cifs/file.c   |    1 +
> > >  fs/ocfs2/mmap.c  |    1 +
> > >  fs/ubifs/file.c  |    4 ++--
> > >  6 files changed, 8 insertions(+), 4 deletions(-)
> > > 
> > > 
> > > diff --git a/fs/9p/vfs_file.c b/fs/9p/vfs_file.c
> > > index c2483e9..aa253f0 100644
> > > --- a/fs/9p/vfs_file.c
> > > +++ b/fs/9p/vfs_file.c
> > > @@ -620,6 +620,7 @@ v9fs_vm_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
> > >  	lock_page(page);
> > >  	if (page->mapping != inode->i_mapping)
> > >  		goto out_unlock;
> > > +	wait_on_stable_page_write(page);
> > >  
> > 
> > Good god thanks, yes please ;-)
> > 
> > >  	return VM_FAULT_LOCKED;
> > >  out_unlock:
> > > diff --git a/fs/afs/write.c b/fs/afs/write.c
> > > index 9aa52d9..39eb2a4 100644
> > > --- a/fs/afs/write.c
> > > +++ b/fs/afs/write.c
> > > @@ -758,7 +758,7 @@ int afs_page_mkwrite(struct vm_area_struct *vma, struct page *page)
> > 
> > afs, is it not a network filesystem? which means that it has it's own emulated none-block-device
> > BDI, registered internally. So if you do need stable pages someone should call
> > bdi_require_stable_pages()
> > 
> > But again since it is a network filesystem I don't see how it is needed, and/or it might be
> > taken care of already.
> > 
> > >  #ifdef CONFIG_AFS_FSCACHE
> > >  	fscache_wait_on_page_write(vnode->cache, page);
> > >  #endif
> > > -
> > > +	wait_on_stable_page_write(page);
> > >  	_leave(" = 0");
> > > -	return 0;
> > > +	return VM_FAULT_LOCKED;
> > >  }
> > > diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
> > 
> > CEPH for sure has it's own "emulated none-block-device BDI". This one is also
> > a pure networking filesystem.
> > 
> > And it already does what it needs to do with wait_on_writeback().
> > 
> > So i do not think you should touch CEPH
> > 
> > > index 6690269..e9734bf 100644
> > > --- a/fs/ceph/addr.c
> > > +++ b/fs/ceph/addr.c
> > > @@ -1208,6 +1208,7 @@ static int ceph_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
> > >  		set_page_dirty(page);
> > >  		up_read(&mdsc->snap_rwsem);
> > >  		ret = VM_FAULT_LOCKED;
> > > +		wait_on_stable_page_write(page);
> > >  	} else {
> > >  		if (ret == -ENOMEM)
> > >  			ret = VM_FAULT_OOM;
> > > diff --git a/fs/cifs/file.c b/fs/cifs/file.c
> > 
> > Cifs also self-BDI network filesystem, but
> > 
> > > index edb25b4..a8770bf 100644
> > > --- a/fs/cifs/file.c
> > > +++ b/fs/cifs/file.c
> > > @@ -2997,6 +2997,7 @@ cifs_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
> > >  	struct page *page = vmf->page;
> > >  
> > >  	lock_page(page);
> > 
> > It waits by locking the page, that's cifs naive way of waiting for writeback
> > 
> > > +	wait_on_stable_page_write(page);
> > 
> > Instead it could do better and not override page_mkwrite at all, and all it needs
> > to do is call bdi_require_stable_pages() at it's own registered BDI
> > 
> 
> Hmm...I don't know...
> 
> I've never been crazy about using the page lock for this, but in the
> absence of a better way to guarantee stable pages, it was what I ended
> up with at the time. cifs_writepages will hold the page lock until
> kernel_sendmsg returns. At that point the TCP layer will have copied
> off the page data so it's safe to release it.
> 
> With this change though, we're going to end up blocking until the
> writeback flag clears, right? And I think that will happen when the
> reply comes in? So, we'll end up blocking for much longer than is
> really necessary in page_mkwrite with this change.

That's a very good point to make-- network FSes can stop the stable-waiting
after the request is sent.  Can I interest you in a new page flag (PG_stable)
that indicates when a page has to be held for stable write?  Along with a
modification to wait_on_stable_page_write that uses the new PG_stable flag
instead of just writeback?  Then, you can clear PG_stable right after the
sendmsg() and release the page for further activity without having to overload
the page lock.

I wrote a patch that does exactly that as part of my work to defer the
integrity checksumming until the last possible instant.  However, I haven't
gotten that part to work yet, so I left the PG_stable patch out of this
submission.  On the other hand, it sounds like you could use it.

--D
> 
> -- 
> Jeff Layton <jlayton@samba.org>
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