Re: [PATCH 2/3] Add shrink_pagecache_parent

From: Andrew Morton
Date: Thu Jan 02 2014 - 18:56:07 EST

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.

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.

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??

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

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