linux-kernel - Re: [PATCH 2/3] Add shrink_pagecache

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Date:	Thu, 2 Jan 2014 15:55:34 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Li Wang <liwang@...ntukylin.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Cong Wang <xiyou.wangcong@...il.com>,
	Zefan Li <lizefan@...wei.com>, Matthew Wilcox <matthew@....cx>,
	Yunchuan Wen <yunchuanwen@...ntukylin.com>,
	Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH 2/3] Add shrink_pagecache_parent

On Mon, 30 Dec 2013 21:45:17 +0800 Li Wang <liwang@...ntukylin.com> 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
problematic.

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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