lkml.org 
[lkml]   [2014]   [Jan]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 2/3] Add shrink_pagecache_parent
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.


\
 
 \ /
  Last update: 2014-01-08 03:41    [W:0.634 / U:0.088 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site