Re: [PATCH 0/6] kernfs: proposed locking and concurrency improvement

[Fox Chen <foxhlchen@gmail.com>]

On Wed, Jan 13, 2021 at 1:17 PM Ian Kent <raven@themaw.net> wrote:
>
> On Mon, 2021-01-11 at 17:02 +0800, Fox Chen wrote:
> > On Mon, Jan 11, 2021 at 4:42 PM Ian Kent <raven@themaw.net> wrote:
> > > On Mon, 2021-01-11 at 15:04 +0800, Fox Chen wrote:
> > > > On Mon, Jan 11, 2021 at 12:20 PM Ian Kent <raven@themaw.net>
> > > > wrote:
> > > > > On Mon, 2021-01-11 at 11:19 +0800, Ian Kent wrote:
> > > > > > On Wed, 2021-01-06 at 10:38 +0800, Fox Chen wrote:
> > > > > > > Hi Ian,
> > > > > > >
> > > > > > > I am rethinking this problem. Can we simply use a global
> > > > > > > lock?
> > > > > > >
> > > > > > >  In your original patch 5, you have a global mutex
> > > > > > > attr_mutex
> > > > > > > to
> > > > > > > protect attr, if we change it to a rwsem, is it enough to
> > > > > > > protect
> > > > > > > both
> > > > > > > inode and attr while having the concurrent read ability?
> > > > > > >
> > > > > > > like this patch I submitted. ( clearly, I missed
> > > > > > > __kernfs_iattrs
> > > > > > > part,
> > > > > > > but just about that idea )
> > > > > > > https://lore.kernel.org/lkml/20201207084333.179132-1-foxhlchen@gmail.com/
> > > > > >
> > > > > > I don't think so.
> > > > > >
> > > > > > kernfs_refresh_inode() writes to the inode so taking a read
> > > > > > lock
> > > > > > will allow multiple processes to concurrently update it which
> > > > > > is
> > > > > > what we need to avoid.
> > > >
> > > > Oh, got it. I missed the inode part. my bad. :(
> > > >
> > > > > > It's possibly even more interesting.
> > > > > >
> > > > > > For example, kernfs_iop_rmdir() and kernfs_iop_mkdir() might
> > > > > > alter
> > > > > > the inode link count (I don't know if that would be the sort
> > > > > > of
> > > > > > thing
> > > > > > they would do but kernfs can't possibly know either). Both of
> > > > > > these
> > > > > > functions rely on the VFS locking for exclusion but the inode
> > > > > > link
> > > > > > count is updated in kernfs_refresh_inode() too.
> > > > > >
> > > > > > That's the case now, without any patches.
> > > > >
> > > > > So it's not so easy to get the inode from just the kernfs
> > > > > object
> > > > > so these probably aren't a problem ...
> > > >
> > > > IIUC only when dop->revalidate, iop->lookup being called, the
> > > > result
> > > > of rmdir/mkdir will be sync with vfs.
> > >
> > > Don't quite get what you mean here?
> > >
> > > Do you mean something like, VFS objects are created on user access
> > > to the file system. Given that user access generally means path
> > > resolution possibly followed by some operation.
> > >
> > > I guess those VFS objects will go away some time after the access
> > > but even thought the code looks like that should happen pretty
> > > quickly after I've observed that these objects stay around longer
> > > than expected. There wouldn't be any use in maintaining a least
> > > recently used list of dentry candidates eligible to discard.
> >
> > Yes, that is what I meant. I think the duration may depend on the
> > current ram pressure. though not quite sure, I'm still digging this
> > part of code.
> >
> > > > kernfs_node is detached from vfs inode/dentry to save ram.
> > > >
> > > > > > I'm not entirely sure what's going on in
> > > > > > kernfs_refresh_inode().
> > > > > >
> > > > > > It could be as simple as being called with a NULL inode
> > > > > > because
> > > > > > the dentry concerned is negative at that point. I haven't had
> > > > > > time to look closely at it TBH but I have been thinking about
> > > > > > it.
> > > >
> > > > um, It shouldn't be called with a NULL inode, right?
> > > >
> > > > inode->i_mode = kn->mode;
> > > >
> > > > otherwise will crash.
> > >
> > > Yes, you're right about that.
> > >
> > > > > Certainly this can be called without a struct iattr having been
> > > > > allocated ... and given it probably needs to remain a pointer
> > > > > rather than embedded in the node the inode link count update
> > > > > can't easily be protected from concurrent updates.
> > > > >
> > > > > If it was ok to do the allocation at inode creation the problem
> > > > > becomes much simpler to resolve but I thought there were
> > > > > concerns
> > > > > about ram consumption (although I don't think that was exactly
> > > > > what
> > > > > was said?).
> > > > >
> > > >
> > > > you meant iattr to be allocated at inode creation time??
> > > > yes, I think so. it's due to ram consumption.
> > >
> > > I did, yes.
> > >
> > > The actual problem is dealing with multiple concurrent updates to
> > > the inode link count, the rest can work.
>
> Umm ... maybe I've been trying to do this in the wrong place all
> along.
>
> You know the inode i_lock looks like the sensible thing to use to
> protect these updates.
>
> Something like this for that last patch should work:
>
> kernfs: use i_lock to protect concurrent inode updates
>
> From: Ian Kent <raven@themaw.net>
>
> The inode operations .permission() and .getattr() use the kernfs node
> write lock but all that's needed is to keep the rb tree stable while
> updating the inode attributes as well as protecting the update itself
> against concurrent changes.
>
> And .permission() is called frequently during path walks and can cause
> quite a bit of contention between kernfs node opertations and path
> walks when the number of concurrant walks is high.
>
> To change kernfs_iop_getattr() and kernfs_iop_permission() to take
> the rw sem read lock instead of the write lock an addtional lock is
> needed to protect against multiple processes concurrently updating
> the inode attributes and link count in kernfs_refresh_inode().
>
> The inode i_lock seems like the sensible thing to use to protect these
> inode attribute updates so use it in kernfs_refresh_inode().
>
> Signed-off-by: Ian Kent <raven@themaw.net>
> ---
>  fs/kernfs/inode.c |   10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c
> index ddaf18198935..e26fa5115821 100644
> --- a/fs/kernfs/inode.c
> +++ b/fs/kernfs/inode.c
> @@ -171,6 +171,7 @@ static void kernfs_refresh_inode(struct kernfs_node *kn, struct inode *inode)
>  {
>         struct kernfs_iattrs *attrs = kn->iattr;
>
> +       spin_lock(inode->i_lock);
>         inode->i_mode = kn->mode;
>         if (attrs)
>                 /*
> @@ -181,6 +182,7 @@ static void kernfs_refresh_inode(struct kernfs_node *kn, struct inode *inode)
>
>         if (kernfs_type(kn) == KERNFS_DIR)
>                 set_nlink(inode, kn->dir.subdirs + 2);
> +       spin_unlock(inode->i_lock);
>  }
>
>  int kernfs_iop_getattr(const struct path *path, struct kstat *stat,
> @@ -189,9 +191,9 @@ int kernfs_iop_getattr(const struct path *path, struct kstat *stat,
>         struct inode *inode = d_inode(path->dentry);
>         struct kernfs_node *kn = inode->i_private;
>
> -       down_write(&kernfs_rwsem);
> +       down_read(&kernfs_rwsem);
>         kernfs_refresh_inode(kn, inode);
> -       up_write(&kernfs_rwsem);
> +       up_read(&kernfs_rwsem);
>
>         generic_fillattr(inode, stat);
>         return 0;
> @@ -281,9 +283,9 @@ int kernfs_iop_permission(struct inode *inode, int mask)
>
>         kn = inode->i_private;
>
> -       down_write(&kernfs_rwsem);
> +       down_read(&kernfs_rwsem);
>         kernfs_refresh_inode(kn, inode);
> -       up_write(&kernfs_rwsem);
> +       up_read(&kernfs_rwsem);
>
>         return generic_permission(inode, mask);
>  }
>

It looks good on my local machine, let me test my benchmark on a big machine. :)

Also, I wonder why i_lock?? what if I use a local spin_lock, will
there be any difference???

static void kernfs_refresh_inode(struct kernfs_node *kn, struct inode *inode)
{
        struct kernfs_iattrs *attrs = kn->iattr;
        static DEFINE_SPINLOCK(inode_lock);

        spin_lock(&inode_lock);
        inode->i_mode = kn->mode;
        if (attrs)
                /*
                 * kernfs_node has non-default attributes get them from
                 * persistent copy in kernfs_node.
                 */
                set_inode_attr(inode, attrs);

        if (kernfs_type(kn) == KERNFS_DIR)
                set_nlink(inode, kn->dir.subdirs + 2);
        spin_unlock(&inode_lock);
}



thanks,
fox