linux-kernel - Re: [PATCH v2] mm/vmscan: count layzfree pages and fix nr_isolated

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Date:   Fri, 24 Apr 2020 13:16:45 +0900
From:   Jaewon Kim <jaewon31.kim@...sung.com>
To:     Minchan Kim <minchan@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>
Cc:     mgorman@...e.de, m.szyprowski@...sung.com, mina86@...a86.com,
        shli@...com, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, jaewon31.kim@...il.com,
        ytk.lee@...sung.com
Subject: Re: [PATCH v2] mm/vmscan: count layzfree pages and fix
 nr_isolated_* mismatch



On 2020년 04월 24일 05:00, Minchan Kim wrote:
> On Thu, Apr 23, 2020 at 12:05:46PM -0400, Johannes Weiner wrote:
>> On Thu, Apr 23, 2020 at 12:16:02PM +0900, Jaewon Kim wrote:
>>>
>>> On 2020년 04월 22일 22:07, Johannes Weiner wrote:
>>>> On Wed, Apr 22, 2020 at 05:48:15PM +0900, Jaewon Kim wrote:
>>>>> @@ -1295,11 +1295,15 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>>>>>  		 */
>>>>>  		if (page_mapped(page)) {
>>>>>  			enum ttu_flags flags = ttu_flags | TTU_BATCH_FLUSH;
>>>>> +			bool lazyfree = PageAnon(page) && !PageSwapBacked(page);
>>>>>  
>>>>>  			if (unlikely(PageTransHuge(page)))
>>>>>  				flags |= TTU_SPLIT_HUGE_PMD;
>>>>> +
>>>>>  			if (!try_to_unmap(page, flags)) {
>>>>>  				stat->nr_unmap_fail += nr_pages;
>>>>> +				if (lazyfree && PageSwapBacked(page))
>>>> This looks pretty strange, until you remember that try_to_unmap()
>>>> could SetPageSwapbacked again.
>>>>
>>>> This might be more obvious?
>>>>
>>>> 			was_swapbacked = PageSwapBacked(page);
>>>> 			if (!try_to_unmap(page, flags)) {
>>>> 				stat->nr_unmap_fail += nr_pages;
>>>> 				if (!was_swapbacked && PageSwapBacked(page))
>>> Hello Johannes, thank you for your comment.
>>>
>>> The name can changed from layzyfree to was_swapbacked.
>>> By the way, did you mean removing PageAnon(page), too? It seems to be OK, though.
>> I can't decide whether PageAnon() makes it clearer or not. But it's
>> not really needed for correctness. So feel free to keep what you had.
> Yub, PageAnon is redundant.
>
>> I would really just at least change bool lazyfree to was_lazyfree,
> It's better.
Thank you both.

Sorry but let me ask again.

Because PageAnon is redundant, let me remove it on checking lazyfree.
Then, I think, was_swapbacked (first suggestion) is better then was_lazyfree (second suggestion),
because it just checks PageSwapBacked(page). And we knows was_swapbacked is used for lazyfree on stat->nr_lazyfree_fail.

If you don't mind let me pick was_swapbacked instead of was_lazyfree
bool was_lazyfree = PageSwapBacked(page);

Thank you

>
>