Re: [PATCH] memcg: handle shmem's swap cache (Was 2.6.26-rc8-mm1

Balbir Singh
2008-07-05T06:49:45Z

KAMEZAWA Hiroyuki wrote:
> On Sat, 05 Jul 2008 11:11:10 +0530
> Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
>
>> KAMEZAWA Hiroyuki wrote:
>>> My swapcache accounting under memcg patch failed to catch tmpfs(shmem)'s one.
>>> Can I test this under -mm tree ?
>>> (If -mm is busy, I'm not in hurry.)
>>> This patch works well in my box.
>>> =
>>> SwapCache handling fix.
>>>
>>> shmem's swapcache behavior is a little different from anonymous's one and
>>> memcg failed to handle it. This patch tries to fix it.
>>>
>>> After this:
>>>
>>> Any page marked as SwapCache is not uncharged. (delelte_from_swap_cache()
>>> delete the SwapCache flag.)
>>>
>>> To check a shmem-page-cache is alive or not we use
>>> page->mapping && !PageAnon(page) instead of
>>> pc->flags & PAGE_CGROUP_FLAG_CACHE.
>>>
>>> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>> Though I am not opposed to this, I do sit up and think if keeping the reference
>> count around could avoid this complexity and from my point, the maintenance
>> overhead of this logic/code (I fear there might be more special cases :( )
>
> yes, to me. but we have to fix..
>
> But I don't like old code's refcnt handling which does
> - increment
> - does this increment was really neccesary ?
> No? ok, decrement it again.
>
> This was much more complex to me than current code.
>

That can be redone -- the moment a page is used by a path, refcnt (increment)
it. Undo the same when the page is no longer in use.

I expect

rmap path to increment/decrement it on mapping
radix-tree (cache's) to do the same

Using a kref we should be able to get this logic right - no?

> And old ones will needs the check at treating swap-cache. (it couldn't but if we want)
>
>> The trade-off is complexity versus the overhead of reference counting.
>>
> refcnt was also very complex ;)

I think that is easier to simply, instead of adding the complex checks we have
right now. refcnt is easier to prove as working correct than the checks.

--
Warm Regards,
Balbir Singh
Linux Technology Center
IBM, ISTL