Re: [PATCH 2/6] VM/NFS: The VM must tell the filesystem when to free reclaimable pages

Trond Myklebust
2010-01-07T04:49:41Z

On Thu, 2010-01-07 at 10:29 +0800, Wu Fengguang wrote:
> On Thu, Jan 07, 2010 at 04:51:10AM +0800, Trond Myklebust wrote:
> > balance_dirty_pages() should really tell the filesystem whether or not it
> > has an excess of actual dirty pages, or whether it would be more useful to
> > start freeing up the unstable writes.
> >
> > Assume that if the number of unstable writes is more than 1/2 the number of
> > reclaimable pages, then we should force NFS to free up the former.
> >
> > Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
> > Acked-by: Jan Kara <jack@suse.cz>
> > ---
> >
> > fs/nfs/write.c | 2 +-
> > include/linux/writeback.h | 5 +++++
> > mm/page-writeback.c | 9 ++++++++-
> > 3 files changed, 14 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/nfs/write.c b/fs/nfs/write.c
> > index 910be28..ee3daf4 100644
> > --- a/fs/nfs/write.c
> > +++ b/fs/nfs/write.c
> > @@ -1417,7 +1417,7 @@ int nfs_commit_unstable_pages(struct address_space *mapping,
> > /* Don't commit yet if this is a non-blocking flush and there are
> > * outstanding writes for this mapping.
> > */
> > - if (wbc->sync_mode != WB_SYNC_ALL &&
> > + if (!wbc->force_commit && wbc->sync_mode != WB_SYNC_ALL &&
> > radix_tree_tagged(&NFS_I(inode)->nfs_page_tree,
> > NFS_PAGE_TAG_LOCKED)) {
> > mark_inode_unstable_pages(inode);
> > diff --git a/include/linux/writeback.h b/include/linux/writeback.h
> > index 76e8903..3fd5c3e 100644
> > --- a/include/linux/writeback.h
> > +++ b/include/linux/writeback.h
> > @@ -62,6 +62,11 @@ struct writeback_control {
> > * so we use a single control to update them
> > */
> > unsigned no_nrwrite_index_update:1;
> > + /*
> > + * The following is used by balance_dirty_pages() to
> > + * force NFS to commit unstable pages.
> > + */
>
> In fact it may be too late to force commit at balance_dirty_pages()
> time: commit takes time and the application has already been blocked.
>
> If not convenient for now, I can make the change -- I'll remove the
> writeback_inodes_wbc() call altogether from balance_dirty_pages().

You could always set the 'for_background' flag instead.

> > + unsigned force_commit:1;
> > };
>
> nfs_commit may be a more newbie friendly name?

We could possibly rename it to something like 'force_nfs_commit', but
the comment above the declaration should really be sufficient.

Trond