On Wed, Feb 26, 2025 at 10:25:12AM +1100, NeilBrown wrote:
> On Tue, 25 Feb 2025, Al Viro wrote:
> > makes simple_lookup() slightly cheaper there.
>
> I think the patch make sense because there is no point keeping negative
> dentries for these filesystems - and positive dentries get an extra
> refcount so DCACHE_DONTCACHE doesn't apply.
>
> But I don't see how this makes simple_lookup() cheaper. It means that
> if someone repeatedly looks up the same non-existent name then
> simple_lookup() will be called more often (because we didn't cache the
> result of the previous time) but otherwise I don't see the relevance to
> simple_lookup(). Am I missing something?
This:
if (!(dentry->d_flags & DCACHE_DONTCACHE)) {
spin_lock(&dentry->d_lock);
dentry->d_flags |= DCACHE_DONTCACHE;
spin_unlock(&dentry->d_lock);
}
IOW, no need to mark that sucker as "don't retain past the moment when
its refcount drops to zero" - they'll all be marked that way since
they'd been created.
Note that we used to switch then to ->d_op that had ->d_delete always
returning true. That had been replaced with setting DCACHE_DONTCACHE;
that was an equivalent transformation. So retention rules have not changed;
the only change here is that this flag doesn't need to be set.
