Re: [PATCH 0/9] Support follow_link in RCU-walk.

[Al Viro <viro@ZenIV.linux.org.uk>]

On Thu, Mar 05, 2015 at 04:21:21PM +1100, NeilBrown wrote:
> Hi Al (and others),
> 
>  I wonder if you could look over this patchset.
>  It allows RCU-walk to follow symlinks in many common cases,
>  thus removing a surprising performance hit caused by using symlinks.
> 
>  The last could of patches make changes to XFS and NFS to support
>  this but I haven't forwarded to the relevant lists yet.
>  If/when the early code meets with approval I'll do that.
> 
>  The first patch almost certainly needs to be changed.  I originally
>  wrote this code when filesystems could see inside nameidata.
>  It is now opaque so the simplest solution was to provide an
>  accessor function.
>  Maybe I should as a 'flags' arg to ->follow_link?? Or have
>  ->follow_link and ->follow_link_rcu ??
>  What do you suggest?

Umm...  Some observations:
	* now ->follow_link() can be called in RCU mode, which means
that it can race with fs shutdown; not a problem, except that now it
joins ->lookup() et.al. in "if some data structure is needed in RCU
case of that, make sure it's not destroyed without an RCU delay somewhere
between the entry into ->kill_sb() and destruction.
	* highmem pages in symlinks: that BS shouldn't be allowed at
all.  Just make sure that at least for those filesystems symlink inodes
get mapping_set_gfp_mask(&inode->i_data, GFP_KERNEL) and be done with that.
	* are you sure that security_inode_follow_link() is OK to call in
RCU mode?
	* what warranties are you giving for the lifetime of strings
passed to nd_set_link()?  Right now it's "should not be freed until the
matching ->put_link()"; what happens for RCU mode?
	* really nasty one: creat(2) on a dangling symlink.  What's to
preserve the last component if you get into that symlink in RCU mode?

TBH, I'm less than fond of passing nameidata to ->follow_link() at all,
flags or no flags.  We could kill current->link_count and
current->total_link_count, replacing them with one void * current->nameidata
and taking counters into struct nameidata itself.  Have places like e.g.
kern_path_locked() do
	struct nameidata nd, *saved = set_nameidata(&nd);
	...
	set_nameidata(saved);
with set_nameidata(p) doing this:
	old = current->nameidata;
	current->nameidata = p;
	if (p) {
		if (!old) {
			p->link_count = 0;
			p->total_link_count = 0;
		} else {
			p->link_count = old->link_count;
			p->total_link_count = old->total_link_count;
		}
	}
	return old;

Then nd_set_link() et.al. would use current->nameidata instead of an
explicitly passed pointer and ->follow_link() instances wouldn't need
that opaque pointer passed to them at all.