Adding namecache entries outside of vfs_lookup and vn_open ?

[Konstantin Belousov]

On Mon, Mar 04, 2019 at 08:24:27AM -0700, Alan Somers wrote:
> On Sun, Mar 3, 2019 at 4:03 AM Konstantin Belousov <kostikbel at gmail.com> wrote:
> > Similar discussion occured some time ago.  I think that the current
> > selection of the cases where namecache entry is created, is optimized
> > for the scenario where extracting large tarball does not largely affect
> > the non-directory elements of the cache.  If you do such extraction,
> > it is unlikely that you will access most of the files shortly.
> 
> I don't understand this objection.  When you extract a tarball full of
> non-empty files, don't you still need to open every file to write its
> contents, creating a namecache entry for each one?
No, you don't.

Typically, when archiver parsed the stream and noted that there is a file
to create with a content, it
- opens the file, and gets the file descriptor returned to usermode.
  Internally, kernel does (vn_open_cred())
	namei() <- this call returns no vnode because the file is non-existent,
		   and does not create negative cache entry, see NOCACHE
		   argument for cn_flags.
	VOP_CREATE() <- creating the file, again not caching
	assign the vnode returned, to the file
- now the process has the descriptor for writes, but namecache entry is
  still not installed.
- content is written, file is closed.