Re: [RFC PATCH v3 0/9] Suppress negative dentry

[Ian Kent <raven@themaw.net>]

On Fri, 2020-05-15 at 15:20 +0800, Chengguang Xu wrote:
> This series adds a new lookup flag LOOKUP_DONTCACHE_NEGATIVE
> to indicate to drop negative dentry in slow path of lookup.
> 
> In overlayfs, negative dentries in upper/lower layers are useless
> after construction of overlayfs' own dentry, so in order to
> effectively reclaim those dentries, specify LOOKUP_DONTCACHE_NEGATIVE
> flag when doing lookup in upper/lower layers.

I've looked at this a couple of times now.

I'm not at all sure of the wisdom of adding a flag to a VFS function
that allows circumventing what a file system chooses to do.

I also do really see the need for it because only hashed negative
dentrys will be retained by the VFS so, if you see a hashed negative
dentry then you can cause it to be discarded on release of the last
reference by dropping it.

So what's different here, why is adding an argument to do that drop
in the VFS itself needed instead of just doing it in overlayfs?

> 
> Patch 1 adds flag LOOKUP_DONTCACHE_NEGATIVE and related logic in vfs
> layer.
> Patch 2 does lookup optimazation for overlayfs.
> Patch 3-9 just adjusts function argument when calling
> lookup_positive_unlocked() and lookup_one_len_unlocked().
> 
> v1->v2:
> - Only drop negative dentry in slow path of lookup.
> 
> v2->v3:
> - Drop negative dentry in vfs layer.
> - Rebase on latest linus-tree(5.7.0-rc5).
> 
> Chengguang Xu (9):
>   fs/dcache: Introduce a new lookup flag LOOKUP_DONTCACHE_NEGATIVE
>   ovl: Suppress negative dentry in lookup
>   cifs: Adjust argument for lookup_positive_unlocked()
>   debugfs: Adjust argument for lookup_positive_unlocked()
>   ecryptfs: Adjust argument for lookup_one_len_unlocked()
>   exportfs: Adjust argument for lookup_one_len_unlocked()
>   kernfs: Adjust argument for lookup_positive_unlocked()
>   nfsd: Adjust argument for lookup_positive_unlocked()
>   quota: Adjust argument for lookup_positive_unlocked()
> 
>  fs/cifs/cifsfs.c      |  2 +-
>  fs/debugfs/inode.c    |  2 +-
>  fs/ecryptfs/inode.c   |  2 +-
>  fs/exportfs/expfs.c   |  2 +-
>  fs/kernfs/mount.c     |  2 +-
>  fs/namei.c            | 14 ++++++++++----
>  fs/nfsd/nfs3xdr.c     |  2 +-
>  fs/nfsd/nfs4xdr.c     |  3 ++-
>  fs/overlayfs/namei.c  |  9 +++++----
>  fs/quota/dquot.c      |  3 ++-
>  include/linux/namei.h |  9 +++++++--
>  11 files changed, 32 insertions(+), 18 deletions(-)
>