Re: [PATCH 2/3] Add shrink_pagecache_parent

[Li Wang <liwang@ubuntukylin.com>]

Hi,

On 01/03/2014 07:55 AM, Andrew Morton wrote:
> On Mon, 30 Dec 2013 21:45:17 +0800 Li Wang <liwang@ubuntukylin.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".
>
Even we have these available, i am afraid it will still introduce
non-negligible overhead due to frequent system calls for a directory
  walking operation, especially under massive small file situations.

>> --- 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.
>
As far as I know, fix me if i am wrong, only when inode has zero
reference count, it will be put into superblock lru list. For most
situations, there is at least a dentry refers to it, so it will not
be on any lru list.

>
> 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).
>
It seems inode_lru_isolate() only called by prune_icache_sb() as
a callback function. Before calling it, the caller has hold
the lock.

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