Re: [PATCH 2/2] exportfs: fix 32-bit nfsd handling of 64-bit inode numbers

["J. Bruce Fields" <bfields@fieldses.org>]

On Wed, Oct 09, 2013 at 11:16:31AM +1100, Dave Chinner wrote:
> On Tue, Oct 08, 2013 at 05:56:56PM -0400, J. Bruce Fields wrote:
> > On Fri, Oct 04, 2013 at 06:15:22PM -0400, J. Bruce Fields wrote:
> > > On Fri, Oct 04, 2013 at 06:12:16PM -0400, bfields wrote:
> > > > On Wed, Oct 02, 2013 at 05:28:14PM -0400, J. Bruce Fields wrote:
> > > > > @@ -268,6 +268,16 @@ static int get_name(const struct path *path, char *name, struct dentry *child)
> > > > >  	if (!dir->i_fop)
> > > > >  		goto out;
> > > > >  	/*
> > > > > +	 * inode->i_ino is unsigned long, kstat->ino is u64, so the
> > > > > +	 * former would be insufficient on 32-bit hosts when the
> > > > > +	 * filesystem supports 64-bit inode numbers.  So we need to
> > > > > +	 * actually call ->getattr, not just read i_ino:
> > > > > +	 */
> > > > > +	error = vfs_getattr_nosec(path, &stat);
> > > > 
> > > > Doh, "path" here is for the parent....  The following works better!
> > > 
> > > By the way, I'm testing this with:
> > > 
> > > 	- create a bunch of nested subdirectories, use
> > > 	  name_to_fhandle_at to get a handle for the bottom directory.
> > > 	- echo 2 >/proc/sys/vm/drop_caches
> > > 	- open_by_fhandle_at on the filehandle
> > > 
> > > But this only actually exercises the reconnect path on the first run
> > > after boot.  Is there something obvious I'm missing here?
> > 
> > Looking at the code....  OK, most of the work of drop_caches is done by
> > shrink_slab_node, which doesn't actually try to free every single thing
> > that it could free--in particular, it won't try to free anything if it
> > thinks there are less than shrinker->batch_size (1024 in the
> > super_block->s_shrink case) objects to free.

(Oops, sorry, that should have been "less than half of
shrinker->batch_size", see below.)

> That's not quite right. Yes, the shrinker won't be called if the
> calculated scan count is less than the batch size, but the left over
> is added back the shrinker scan count to carry over to the next call
> to the shrinker. Hence if you repeated call the shrinker on a small
> cache with a large batch size, it will eventually aggregate the scan
> counts to over the batch size and trim the cache....

No, in shrink_slab_count, we do this:

	if (total_scan > max_pass * 2)
		total_scan = max_pass * 2;

	while (total_scan >= batch_size) {
		...
	}

where max_pass is the value returned from count_objects.  So as long as
count_objects returns less than half batch_size, nothing ever happens.

(I wonder if that check's correct?  The "forever" in the comment above
it seems wrong at least.)

--b.