Re: [REPOST PATCH v4 2/5] kernfs: use VFS negative dentry caching

[Eric W. Biederman]

Ian Kent <raven@xxxxxxxxxx> writes:

> On Fri, 2021-06-04 at 09:28 -0500, Eric W. Biederman wrote:
>> Ian Kent <raven@xxxxxxxxxx> writes:
>> 
>> > On Thu, 2021-06-03 at 17:02 -0500, Eric W. Biederman wrote:
>> > > Miklos Szeredi <miklos@xxxxxxxxxx> writes:
>> > > 
>> > > > On Thu, 3 Jun 2021 at 19:26, Eric W. Biederman < 
>> > > > ebiederm@xxxxxxxxxxxx> wrote:
>> > > > > 
>> > > > > Ian Kent <raven@xxxxxxxxxx> writes:
>> > > > > 
>> > > > > > If there are many lookups for non-existent paths these
>> > > > > > negative
>> > > > > > lookups
>> > > > > > can lead to a lot of overhead during path walks.
>> > > > > > 
>> > > > > > The VFS allows dentries to be created as negative and
>> > > > > > hashed,
>> > > > > > and caches
>> > > > > > them so they can be used to reduce the fairly high overhead
>> > > > > > alloc/free
>> > > > > > cycle that occurs during these lookups.
>> > > > > > 
>> > > > > > Signed-off-by: Ian Kent <raven@xxxxxxxxxx>
>> > > > > > ---
>> > > > > >  fs/kernfs/dir.c |   55 +++++++++++++++++++++++++++++++++--
>> > > > > > ----
>> > > > > > ----------------
>> > > > > >  1 file changed, 33 insertions(+), 22 deletions(-)
>> > > > > > 
>> > > > > > diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
>> > > > > > index 4c69e2af82dac..5151c712f06f5 100644
>> > > > > > --- a/fs/kernfs/dir.c
>> > > > > > +++ b/fs/kernfs/dir.c
>> > > > > > @@ -1037,12 +1037,33 @@ static int
>> > > > > > kernfs_dop_revalidate(struct
>> > > > > > dentry *dentry, unsigned int flags)
>> > > > > >       if (flags & LOOKUP_RCU)
>> > > > > >               return -ECHILD;
>> > > > > > 
>> > > > > > -     /* Always perform fresh lookup for negatives */
>> > > > > > -     if (d_really_is_negative(dentry))
>> > > > > > -             goto out_bad_unlocked;
>> > > > > > +     mutex_lock(&kernfs_mutex);
>> > > > > > 
>> > > > > >       kn = kernfs_dentry_node(dentry);
>> > > > > > -     mutex_lock(&kernfs_mutex);
>> > > > > 
>> > > > > Why bring kernfs_dentry_node inside the mutex?
>> > > > > 
>> > > > > The inode lock of the parent should protect negative to
>> > > > > positive
>> > > > > transitions not the kernfs_mutex.  So moving the code inside
>> > > > > the mutex looks unnecessary and confusing.
>> > > > 
>> > > > Except that d_revalidate() may or may not be called with parent
>> > > > lock
>> > > > held.
>> > 
>> > Bringing the kernfs_dentry_node() inside taking the mutex is
>> > probably
>> > wasteful, as you say, oddly the reason I did it that conceptually
>> > it
>> > makes sense to me since the kernfs node is being grabbed. But it
>> > probably isn't possible for a concurrent unlink so is not
>> > necessary.
>> > 
>> > Since you feel strongly about I can change it.
>> > 
>> > > 
>> > > I grant that this works because kernfs_io_lookup today holds
>> > > kernfs_mutex over d_splice_alias.
>> > 
>> > Changing that will require some thought but your points about
>> > maintainability are well taken.
>> > 
>> > > 
>> > > The problem is that the kernfs_mutex only should be protecting
>> > > the
>> > > kernfs data structures not the vfs data structures.
>> > > 
>> > > Reading through the code history that looks like a hold over from
>> > > when
>> > > sysfs lived in the dcache before it was reimplemented as a
>> > > distributed
>> > > file system.  So it was probably a complete over sight and
>> > > something
>> > > that did not matter.
>> > > 
>> > > The big problem is that if the code starts depending upon the
>> > > kernfs_mutex (or the kernfs_rwsem) to provide semantics the rest
>> > > of
>> > > the
>> > > filesystems does not the code will diverge from the rest of the
>> > > filesystems and maintenance will become much more difficult.
>> > > 
>> > > Diverging from other filesystems and becoming a maintenance pain
>> > > has
>> > > already been seen once in the life of sysfs and I don't think we
>> > > want
>> > > to
>> > > go back there.
>> > > 
>> > > Further extending the scope of lock, when the problem is that the
>> > > locking is causing problems seems like the opposite of the
>> > > direction
>> > > we
>> > > want the code to grow.
>> > > 
>> > > I really suspect all we want kernfs_dop_revalidate doing for
>> > > negative
>> > > dentries is something as simple as comparing the timestamp of the
>> > > negative dentry to the timestamp of the parent dentry, and if the
>> > > timestamp has changed perform the lookup.  That is roughly what
>> > > nfs does today with negative dentries.
>> > > 
>> > > The dentry cache will always lag the kernfs_node data structures,
>> > > and
>> > > that is fundamental.  We should take advantage of that to make
>> > > the
>> > > code
>> > > as simple and as fast as we can not to perform lots of work that
>> > > creates
>> > > overhead.
>> > > 
>> > > Plus the kernfs data structures should not change much so I
>> > > expect
>> > > there will be effectively 0 penalty in always performing the
>> > > lookup
>> > > of a
>> > > negative dentry when the directory itself has changed.
>> > 
>> > This sounds good to me.
>> > 
>> > In fact this approach should be able to be used to resolve the
>> > potential race Miklos pointed out in a much simpler way, not to
>> > mention the revalidate simplification itself.
>> > 
>> > But isn't knowing whether the directory has changed harder to
>> > do than checking a time stamp?
>> > 
>> > Look at kernfs_refresh_inode() and it's callers for example.
>> > 
>> > I suspect that would require bringing back the series patch to use
>> > a generation number to identify directory changes (and also getting
>> > rid of the search in revalidate).
>> 
>> In essence it is a simple as looking at a sequence number or a
>> timestamp
>> to detect the directory has changed.
>
> Yes, both Miklos and Al suggested using a simple revision to detect
> changes to the parent. I did that early on and I don't think I grokked
> what Al recommended and ended up with something more complex than was
> needed. So I dropped it because I wanted to keep the changes to a
> minimum.
>
> But a quick test, bringing that patch back, and getting rid of the
> search in revalidate works well. It's as effective at eliminating
> contention I saw with d_alloc_parallel() for the case of a lot of
> deterministic accesses to the same non-existent file as the racy
> search method I had there, perhaps a bit better, it's certainly
> more straight forward.
>
>> 
>> In practice there are always details that make things more
>> complicated.
>> 
>> I was actually wondering if the approach should be to have an seqlock
>> around an individual directories rbtree.  I think that would give a
>> lot
>> of potential for rcu style optimization during lookups.
>
> Yeah, it's tempting, but another constraint I had is to not increase
> the size of the kernfs_node struct (Greg and Tejun) and there's a
> hole in the node union variant kernfs_elem_dir at least big enough
> for sizeof(pointer) so I can put the revision there. And, given the
> simplification in revalidate, as well as that extra code being pretty
> straight forward itself, it's not too bad from the minimal change
> POV.
>
> So I'd like to go with using a revision for now.

No objection from me.

Eric