Re: [PATCH 1/6] VFS: Ensure that writeback_single_inode() commits unstable writes

[Wu Fengguang <fengguang.wu-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

On Thu, Jan 07, 2010 at 11:10:22PM +0800, Trond Myklebust wrote:
> On Thu, 2010-01-07 at 22:56 +0800, Wu Fengguang wrote: 
> > On Thu, Jan 07, 2010 at 12:38:02PM +0800, Myklebust, Trond wrote:
> > 
> > > > > diff --git a/fs/nfs/write.c b/fs/nfs/write.c
> > > > > index d171696..910be28 100644
> > > > > --- a/fs/nfs/write.c
> > > > > +++ b/fs/nfs/write.c
> > > > > @@ -441,7 +441,7 @@ nfs_mark_request_commit(struct nfs_page *req)
> > > > >     spin_unlock(&inode->i_lock);
> > > > >     inc_zone_page_state(req->wb_page, NR_UNSTABLE_NFS);
> > > > >     inc_bdi_stat(req->wb_page->mapping->backing_dev_info, BDI_RECLAIMABLE);
> > > > > -   __mark_inode_dirty(inode, I_DIRTY_DATASYNC);
> > > > > +   mark_inode_unstable_pages(inode);
> > > >
> > > > Then we shall mark I_DIRTY_DATASYNC on other places that extend i_size.
> > > 
> > > Why? The NFS client itself shouldn't ever set I_DIRTY_DATASYNC after
> > > this patch is applied. We won't ever need it.
> > > 
> > > If the VM or VFS is doing it, then they ought to be fixed: there is no
> > > reason to assume that all filesystems need to sync their inodes on
> > > i_size changes.
> > 
> > Sorry, one more question.
> > 
> > It seems to me that you are replacing
> > 
> >         I_DIRTY_DATASYNC => write_inode()
> > with
> >         I_UNSTABLE_PAGES => commit_unstable_pages()
> > 
> > Is that change for the sake of clarity? Or to fix some problem?
> > (This patch does fix some problems, but do they inherently require
> > the above change?)
> 
> As I said previously, the write_inode() call is done _before_ you sync
> the dirty pages to the server, whereas commit_unstable_pages() wants to
> be done _after_ syncing. So the two are not the same, and we cannot
> replace commit_unstable_pages() with write_inode().

This is the ordering:

0       do_writepages()
1       if (I_DIRTY_SYNC | I_DIRTY_DATASYNC)
2               write_inode()
3       if (wait)
4               filemap_fdatawait()
5       if (I_UNSTABLE_PAGES)
6               commit_unstable_pages()

The page is synced to NFS server in line 0.

The only difference is write_inode() is called before filemap_fdatawait(),
while commit_unstable_pages() is called after it.

Note that filemap_fdatawait() will only be called on WB_SYNC_ALL, so I
still cannot understand the difference..

> Replacing I_DIRTY_DATASYNC with I_UNSTABLE_PAGES is more for the sake of
> clarity. The difference between the two is that in the I_UNSTABLE_PAGES
> case, the inode itself isn't actually dirty; it just contains pages that
> are not guaranteed to be on permanent storage until we commit.

And I_UNSTABLE_PAGES is necessary for calling commit_unstable_pages() :)

Thanks,
Fengguang
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