Re: [PATCH v2] kernfs: allocate inodes and dentry outside kernfs_rwsem

[Shakeel Butt <shakeel.butt@linux.dev>]

On Fri, Oct 16, 2026 at 10:33:20AM -1000, Tejun Heo wrote:
> Hello, Shakeel.
> 
> On Fri, Oct 16, 2026 at 09:43:45AM -0700, Shakeel Butt wrote:
> > Allocate the inode with the lock dropped.  kernfs_iget() first looks for
> > an existing inode.  If there is none, it pins the node, drops the lock,
> > allocates, takes the lock again, and adds the inode only if the node is
> > still active.  So __kernfs_remove() still finds every inode it must
> > clear i_nlink on, as commit dc1a0cc95737 ("kernfs: don't lose
> > IN_DELETE_SELF when decoding a file handle") needs.
> 
> Would it be simpler to allocate the inode before taking kernfs_rwsem and
> free it if it ends up unused, as 8b64f32fee95 ("kernfs: allocate the new
> name outside kernfs_rwsem") and 5ad03af18434 ("kernfs: allocate the open
> node outside the open file mutex") do? Then the lock doesn't have to be
> dropped, and there's no need for the pin and the active recheck. The
> unused cases (negative lookups, inactive nodes, nodes which already have
> an inode) are exceptions, and an inode alloc and free isn't expensive.

Thanks for taking a look and yes, it is much better and simpler.

> 
> The helper would then just run under kernfs_rwsem. Maybe
> kernfs_get_inode_locked() would be a better name for it?

Ack

> 
> kernfs_fill_super() can preallocate the same way, which gets rid of the
> last inode allocation under kernfs_rwsem.

Ack

> cgroup_may_write() allocates
> the inode under cgroup_mutex and could allocate before grabbing the
> mutex too.

Let me send this one as a separate patch.

> 
> > kernfs_get_parent_dentry() also held the lock across d_obtain_alias(),
> > which allocates a dentry.  Hold it only around kernfs_iget(), as
> > __kernfs_fh_to_dentry() already does.
> 
> This also adds the active test to ->get_parent. A parent which is being
> removed and has no inode now gets -ESTALE instead of a new inode with
> i_nlink 1, which closes the window dc1a0cc95737 ("kernfs: don't lose
> IN_DELETE_SELF when decoding a file handle") left open. Can you mention
> it in the description?

Yes, I will do.

Thanks again for the review.