Re: [PATCH 0/7 RFC] New APIs for name lookup and lock for directory operations

[Amir Goldstein]

On Mon, Jul 21, 2025 at 10:46 AM NeilBrown <neil@xxxxxxxxxx> wrote:
>
> Hi,
>
>  these patches (against vfs.all) primarily introduce new APIs for
>  preparing dentries for create, remove, rename.  The goal is to
>  centralise knowledge of how we do locking (currently by locking the
>  directory) so that we can eventually change the mechanism (e.g.  to
>  locking just the dentry).
>
>  Naming is difficult and I've changed my mind several times. :-)

Indeed it is.
I generally like the done_ approach that you took.
Few minor naming comments follow.

>
>  The basic approach is to return a dentry which can be passed to
>  vfs_create(), vfs_unlink() etc, and subsequently to release that
>  dentry.  The closest analogue to this in the VFS is kern_path_create()
>  which is paired with done_path_create(), though there is also
>  kern_path_locked() which is paired with explicit inode_unlock() and
>  dput().  So my current approach uses "done_" for finishing up.
>
>  I have:
>    dentry_lookup() dentry_lookup_noperm() dentry_lookup_hashed()

As I wrote on the patch that introduces them I find dentry_lookup_hashed()
confusing because the dentry is not hashed (only the hash is calculated).

Looking at another precedent of _noperm() vfs API we have:

vfs_setxattr()
  __vfs_setxattr_locked()
    __vfs_setxattr_noperm()
      __vfs_setxattr()

Do I'd say for lack of better naming __dentry_lookup() could makes sense
for the bare lock&dget and it could also be introduced earlier along with
introducing done_dentry_lookup()

>    dentry_lookup_killable()
>  paired with
>    done_dentry_lookup()
>
>  and also
>    rename_lookup() rename_lookup_noperm() rename_lookup_hashed()
>  paired with
>    done_rename_lookup()
>  (these take a "struct renamedata *" to which some qstrs are added.
>
>  There is also "dentry_lock_in()" which is used instead of
>  dentry_lookup() when you already have the dentry and want to lock it.
>  So you "lock" it "in" a given parent.  I'm not very proud of this name,
>  but I don't want to use "dentry_lock" as I want to save that for
>  low-level locking primitives.

Very strange name :)

What's wrong with dentry_lock_parent()?

Although I think that using the verb _lock_ for locking and dget is
actively confusing, so something along the lines of
resume_dentry_lookup()/dentry_lookup_reacquire() might serve the
readers of the code better.

>
>  There is also done_dentry_lookup_return() which doesn't dput() the
>  dentry but returns it instread.  In about 1/6 of places where I need
>  done_dentry_lookup() the code makes use of the dentry afterwards.  Only
>  in half the places where done_dentry_lookup_return() is used is the
>  returned value immediately returned by the calling function.  I could
>  do a dget() before done_dentry_lookup(), but that looks awkward and I
>  think having the _return version is justified.  I'm happy to hear other
>  opinions.

The name is not very descriptive IMO, but I do not have a better suggestion.
Unless you can describe it for the purpose that it is used for, e.g.
yeild_dentry_lookup() that can be followed with resume_dentry_lookup(),
but I do not know if those are your intentions for the return API.

Thanks,
Amir.

>
>  In order for this dentry-focussed API to work we need to have the
>  dentry to unlock.  vfs_rmdir() currently consumes the dentry on
>  failure, so we don't have it unless we clumsily keep a copy.  So an
>  early patch changes vfs_rmdir() to both consume the dentry and drop the
>  lock on failure.
>
>  After these new APIs are refined, agreed, and applied I will have a
>  collection of patches to roll them out throughout the kernel.  Then we
>  can start/continue discussing a new approach to locking which allows
>  directory operations to proceed in parallel.
>
>  If you want a sneak peek at some of this future work - for context
>  mostly - my current devel code is at https://github.com/neilbrown/linux.git
>  in a branch "pdirops".  Be warned that a lot of the later code is under
>  development, is known to be wrong, and doesn't even compile.  Not today
>  anyway.  The rolling out of the new APIs is fairly mature though.
>
>  Please review and suggest better names, or tell me that my choices are adequate.
>  And find the bugs in the code too :-)
>
>  I haven't cc:ed the maintains of the non-VFS code that the patches
>  touch.  I can do that once the approach and names have been approved.
>
> Thanks,
> NeilBrown
>
>
>  [PATCH 1/7] VFS: unify old_mnt_idmap and new_mnt_idmap in renamedata
>  [PATCH 2/7] VFS: introduce done_dentry_lookup()
>  [PATCH 3/7] VFS: Change vfs_mkdir() to unlock on failure.
>  [PATCH 4/7] VFS: introduce dentry_lookup() and friends
>  [PATCH 5/7] VFS: add dentry_lookup_killable()
>  [PATCH 6/7] VFS: add rename_lookup()
>  [PATCH 7/7] VFS: introduce dentry_lock_in()
>