Re: [PATCH v3 01/16] mm: use put_page to free page instead of putback_lru_page

Balbir Singh
2016-04-05T03:10:53Z

On 04/04/16 16:01, Minchan Kim wrote:
> On Mon, Apr 04, 2016 at 03:53:59PM +1000, Balbir Singh wrote:
>>
>> On 30/03/16 18:12, Minchan Kim wrote:
>>> Procedure of page migration is as follows:
>>>
>>> First of all, it should isolate a page from LRU and try to
>>> migrate the page. If it is successful, it releases the page
>>> for freeing. Otherwise, it should put the page back to LRU
>>> list.
>>>
>>> For LRU pages, we have used putback_lru_page for both freeing
>>> and putback to LRU list. It's okay because put_page is aware of
>>> LRU list so if it releases last refcount of the page, it removes
>>> the page from LRU list. However, It makes unnecessary operations
>>> (e.g., lru_cache_add, pagevec and flags operations. It would be
>>> not significant but no worth to do) and harder to support new
>>> non-lru page migration because put_page isn't aware of non-lru
>>> page's data structure.
>>>
>>> To solve the problem, we can add new hook in put_page with
>>> PageMovable flags check but it can increase overhead in
>>> hot path and needs new locking scheme to stabilize the flag check
>>> with put_page.
>>>
>>> So, this patch cleans it up to divide two semantic(ie, put and putback).
>>> If migration is successful, use put_page instead of putback_lru_page and
>>> use putback_lru_page only on failure. That makes code more readable
>>> and doesn't add overhead in put_page.
>> So effectively when we return from unmap_and_move() the page is either
>> put_page or putback_lru_page() and the page is gone from under us.
> I didn't get your point.
> Could you elaborate it more what you want to say about this patch?

I was just adding to my understanding of this change based on your changelog.
My understanding is that we take the extra reference in isolate_lru_page()
but by the time we return from unmap_and_move() we drop the extra reference

Balbir Singh