On Tue, 1 May 2012 22:15:04 +0200
Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> On Tue, May 01, 2012 at 12:02:46PM -0700, Andrew Morton wrote:
> > On Tue, 1 May 2012 10:41:50 +0200
> > Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> >
> > > --- a/fs/inode.c
> > > +++ b/fs/inode.c
> > > @@ -544,8 +544,7 @@ static void evict(struct inode *inode)
> > > if (op->evict_inode) {
> > > op->evict_inode(inode);
> > > } else {
> > > - if (inode->i_data.nrpages)
> > > - truncate_inode_pages(&inode->i_data, 0);
> > > + truncate_inode_pages(&inode->i_data, 0);
> >
> > Why did we lose this optimisation?
>
> For inodes with only shadow pages remaining in the tree, because there
> is no separate counter for them. Otherwise, we'd leak the tree nodes.
>
> I had mapping->nrshadows at first to keep truncation conditional, but
> thought that using an extra word per cached inode would be worse than
> removing this optimization. There is not too much being done when the
> tree is empty.
>
> Another solution would be to include the shadows count in ->nrpages,
> but filesystems use this counter for various other purposes.
>
> Do you think it's worth reconsidering?
It doesn't sound like it's worth adding ->nrshadows for only that
reason.
That's a pretty significant alteration in the meaning of ->nrpages.
Did this not have any other effects?
What does truncate do? I assume it invalidates shadow page entries in
the radix tree? And frees the radix-tree nodes?
The patchset will make lookups slower in some (probably obscure)
circumstances, due to the additional radix-tree nodes.
I assume that if a pagecache lookup encounters a radix-tree node which
contains no real pages, the search will terminate at that point? We
don't pointlessly go all the way down to the leaf nodes?
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