On Thu, 23 May 2013 22:30:10 +0000
"Myklebust, Trond" <Trond.Myklebust@xxxxxxxxxx> wrote:
> On Thu, 2013-05-23 at 18:24 -0400, Jeff Layton wrote:
> > On Thu, 23 May 2013 17:53:41 -0400
> > Scott Mayhew <smayhew@xxxxxxxxxx> wrote:
> >
> > > Currently nfs_updatepage allows a write to be extended to cover a full
> > > page only if we don't have a byte range lock on the file... but if we've
> > > got the whole file locked, then we should be allowed to extend the
> > > write.
> > >
> > > Signed-off-by: Scott Mayhew <smayhew@xxxxxxxxxx>
> > > ---
> > > fs/nfs/write.c | 7 +++++--
> > > 1 file changed, 5 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/fs/nfs/write.c b/fs/nfs/write.c
> > > index a2c7c28..f35fb4f 100644
> > > --- a/fs/nfs/write.c
> > > +++ b/fs/nfs/write.c
> > > @@ -908,13 +908,16 @@ int nfs_updatepage(struct file *file, struct page *page,
> > > file->f_path.dentry->d_name.name, count,
> > > (long long)(page_file_offset(page) + offset));
> > >
> > > - /* If we're not using byte range locks, and we know the page
> > > + /* If we're not using byte range locks (or if the range of the
> > > + * lock covers the entire file), and we know the page
> > > * is up to date, it may be more efficient to extend the write
> > > * to cover the entire page in order to avoid fragmentation
> > > * inefficiencies.
> > > */
> > > if (nfs_write_pageuptodate(page, inode) &&
> > > - inode->i_flock == NULL &&
> > > + (inode->i_flock == NULL ||
> > > + (inode->i_flock->fl_start == 0 &&
> > > + inode->i_flock->fl_end == OFFSET_MAX)) &&
> > > !(file->f_flags & O_DSYNC)) {
> > > count = max(count + offset, nfs_page_length(page));
> > > offset = 0;
> >
> > Sounds like a reasonable proposition, but I think you might need to do
> > more vetting of the locks...
> >
> > For instance, does it make sense to do this if it's a F_RDLCK? Also,
> > you're only looking at the first lock in the i_flock list. Might it
> > make more sense to walk the list and see whether the page might be
> > entirely covered by a lock that doesn't extend over the whole file?
> >
>
> I'm guessing that the answer is to both these questions are "no":
> - Anybody who is writing while holding a F_RDLCK is likely doing
> something wrong.
Right, so I think we ought to be conservative here and not extend the
write if this is an F_RDLCK.
> - Walking the lock list on every write can quickly get painful if we
> have lots of small locks.
>
True, but it's probably still preferable to do that than to do a bunch
of small I/Os to the server. But, that's an optimization that can be
done later. Hardly anyone does real byte-range locking so I'm fine with
this approach for now.
> However it may make a lot of sense to look at whether or not we hold a
> NFSv4 write delegation.
>
Yes, that would be a good thing too. Having a helper function like you
suggested should make it easier to encapsulate that logic sanely.
--
Jeff Layton <jlayton@xxxxxxxxxx>
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