On Thu, Jan 02, 2014 at 03:55:34PM -0800, Andrew Morton wrote:

On Mon, 30 Dec 2013 21:45:17 +0800 Li Wang [off-list ref] wrote:

quoted

Analogous to shrink_dcache_parent except that it collects inodes.
It is not very appropriate to be put in dcache.c, but d_walk can only
be invoked from here.

Please cc Dave Chinner on future revisions.  He be da man.

The overall intent of the patchset seems reasonable and I agree that it
can't be efficiently done from userspace with the current kernel API. 
We *could* do it from userspace by providing facilities for userspace to
query the VFS caches: "is this pathname in the dentry cache" and "is
this inode in the inode cache".

quoted

--- a/fs/dcache.c
+++ b/fs/dcache.c

@@ -1318,6 +1318,42 @@ void shrink_dcache_parent(struct dentry *parent)
 }
 EXPORT_SYMBOL(shrink_dcache_parent);
 
+static enum d_walk_ret gather_inode(void *data, struct dentry *dentry)
+{
+	struct list_head *list = data;
+	struct inode *inode = dentry->d_inode;
+
+	if ((inode == NULL) || ((!inode_owner_or_capable(inode)) &&
+				(!capable(CAP_SYS_ADMIN))))
+		goto out;
+	spin_lock(&inode->i_lock);
+	if ((inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW)) ||

It's unclear what rationale lies behind this particular group of tests.

quoted

+		(inode->i_mapping->nrpages == 0) ||
+		(!list_empty(&inode->i_lru))) {

arg, the "Inode locking rules" at the top of fs/inode.c needs a
refresh, I suspect.  It is too vague.

Yes, it probably does need work.

Formally, inode->i_lru is protected by
i_sb->s_inode_lru->node[nid].lock, not by ->i_lock.  I guess you can
just do a list_lru_add() and that will atomically add the inode to your
local list_lru if ->i_lru wasn't being used for anything else.

There is no such thing as a "local" list_lru. If you need to put an
object on a local list, then just use a local struct list_head.
That's how we do dispose lists for the objects being removed from
the LRU...

However, the only way you can check if the i_lru is not in use is to
hold the relevant LRU lock, and that's something that should not be
directly accessed - the internal locking of the LRU is private,
subject to change and as such is only accessible in th places that
it is explicitly exposed. i.e. the ->isolate callback.

I *think* that your use of i_lock works OK, because code which fiddles
with i_lru and s_inode_lru also takes i_lock.  However we need to
decide which is the preferred and official lock.  ie: what is the
design here??

THe LRU lock nests inside the i_lock. The i_lock does not provide
exclusive access to i_lru if the inode is on the LRU; LRU list
manipulations can modify i_lru (e.g. removing an adjacent inode in
the LRU list) without holding i_lock....

However...  most inodes will be on an LRU list, won't they?  Doesn't
this reuse of i_lru mean that many inodes will fail to be processed? 
If so, we might need to add a new list_head to the inode, which will be
problematic.

Yes, yes, and yes, adding a new list head to the struct inode for
such an uncommon corner case is a non-starter.

Aside: inode_lru_isolate() fiddles directly with inode->i_lru without
taking i_sb->s_inode_lru->node[nid].lock.  Why doesn't this make a
concurrent s_inode_lru walker go oops??  Should we be using
list_lru_del() in there? 

No, inode_lru_isoalte() is called with the lru lock held. The
specific list lock is passed as the lru_lock parameter, so it can be
droppped if a blocking operation needs to be done to prepare the
object for isolation.  So, calling list_lru_del() will deadlock on
the LRU lock.

(which should have been called list_lru_del_init(), sigh).

That implies that removing the object from the LRU without
initialising the object being removed is a valid thing to do. It's
not - the lru_list code requires that an object not on an LRU is in
an intialised state so that list_empty() checks work correctly. i.e
list_lru_del(object); list_lru_add(object); needs to work, and that
is non-negotiable. So, no need for suffixes to define different
behaviours - there can be only one...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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