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SubjectRe: [RFD 1/4] Pass no useless nameidata to the create, lookup, and permission IOPs
In message <>, writes:
> The create, lookup, and permission inode operations are all passed a
> full nameidata. This is unfortunate because in nfsd and the mqueue
> filesystem, we must instantiate a struct nameidata but cannot provide
> all of the same information that a regular lookup would provide. The
> unused fields take up space on the stack, but more importantly, it is
> not obvious which fields have meaningful values and which don't, and so
> things might easily break.
> This patch introduces struct nameidata2 with only the fields that make
> sense independent of an actual lookup, and uses that struct in those
> places where a full nameidat is not needed.

I agree w/ Trond that a better name is needed other than 'nameidata2',
esp. for something that's a sub-structure (perhaps start it with a '__'?)

These changes would probably help stackable file systems (e.g., eCryptfs and
esp. Unionfs) a lot, b/c stackable f/s often call the lower f/s to lookup
files and such; and in most cases, we just need to pass the intent down, not
the full VFS-level state info.

> +/**
> + * Fields shared between nameidata and nameidata2 -- nameidata2 could
> + * simply be embedded in nameidata, but then the vfs code would become
> + * cluttered with dereferences.
> + */
> +#define __NAMEIDATA2 \
> + struct dentry *dentry; \
> + struct vfsmount *mnt; \
> + unsigned int flags; \
> + \
> + union { \
> + struct open_intent open; \
> + } intent;

Perhaps it is also time to put the dentry + mnt into a single struct path?
It's a small change, but it emphasizes that the two items here, dentry+mnt,
really define a single path to be passed around:

#define __NAMEIDATA \
struct path path; \
unsigned int flags; \

Of course, you'll have to change instances of nd->dentry to nd->path.dentry
and so on.

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