On Thu, Jan 02, 2014 at 03:55:34PM -0800, Andrew Morton wrote:
> On Mon, 30 Dec 2013 21:45:17 +0800 Li Wang <liwang@xxxxxxxxxxxxxxx> wrote:
>
> > 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".
>
> > --- 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.
>
> > + (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@xxxxxxxxxxxxx
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