Subject: Re: [PATCH v2 1/2] mm: Uncharge poisoned pages
To: Balbir Singh <bsingharora@gmail.com>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
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Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>
From: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Date: Fri, 28 Apr 2017 11:32:31 +0200
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On 26/04/2017 10:59, Balbir Singh wrote:
> On Wed, 2017-04-26 at 04:46 +0000, Naoya Horiguchi wrote:
>> On Wed, Apr 26, 2017 at 01:45:00PM +1000, Balbir Singh wrote:
>>>>>>  static int delete_from_lru_cache(struct page *p)
>>>>>>  {
>>>>>> +	if (memcg_kmem_enabled())
>>>>>> +		memcg_kmem_uncharge(p, 0);
>>>>>> +
>>>>>
>>>>> The changelog is not quite clear, so we are uncharging a page using
>>>>> memcg_kmem_uncharge for a page in swap cache/page cache?
>>>>
>>>> Hi Balbir,
>>>>
>>>> Yes, in the normal page lifecycle, uncharge is done in page free time.
>>>> But in memory error handling case, in-use pages (i.e. swap cache and page
>>>> cache) are removed from normal path and they don't pass page freeing code.
>>>> So I think that this change is to keep the consistent charging for such a case.
>>>
>>> I agree we should uncharge, but looking at the API name, it seems to
>>> be for kmem pages, why are we not using mem_cgroup_uncharge()? Am I missing
>>> something?
>>
>> Thank you for pointing out.
>> Actually I had the same question and this surely looks strange.
>> But simply calling mem_cgroup_uncharge() here doesn't work because it
>> assumes that page_refcount(p) == 0, which is not true in hwpoison context.
>> We need some other clearer way or at least some justifying comment about
>> why this is ok.
>>
> 
> We should call mem_cgroup_uncharge() after isolate_lru_page()/put_page().

Thanks for the review Naoya and Balbir,

I changed the patch to call mem_cgroup_uncharge() once
isolate_lru_page() succeeded, but before calling put_page().
It seems to work fine.

> We could check if page_count() is 0 or force if required (!MF_RECOVERED &&
> !MF_DELAYED). We could even skip the VM_BUG_ON if the page is poisoned.

This doesn't seem to be needed. Am I still missing something here ?

Cheers,
Laurent.