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

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Date:   Thu, 23 Apr 2020 12:05:46 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Jaewon Kim <jaewon31.kim@...sung.com>
Cc:     minchan@...nel.org, 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 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.

I would really just at least change bool lazyfree to was_lazyfree,
otherwise it seems a bit confusing. was_lazyfree makes it a bit
clearer that we expect try_to_unmap() might change the state.

> >> +					stat->nr_lazyfree_fail += nr_pages;
> >>  				goto activate_locked;
> > Or at least was_lazyfree.
> Sorry but I'm confused.
> I think you meant additional comment to previous your comment
> rather than you wanted to rename stat->nr_lazyfree_fail to stat->was_lazyfree.

No just the bool variable, the stat one seems fine to me.

> >> @@ -1491,8 +1495,8 @@ unsigned long reclaim_clean_pages_from_list(struct zone *zone,
> >>  		.priority = DEF_PRIORITY,
> >>  		.may_unmap = 1,
> >>  	};
> >> -	struct reclaim_stat dummy_stat;
> >> -	unsigned long ret;
> >> +	struct reclaim_stat stat;
> >> +	unsigned long reclaimed;
> > nr_reclaimed would be better.
> I will add nr_ prefix on next patch.

Thanks!

> > I also prefer keeping dummy_stat, since that's still what it is.
> This patch uses stat.nr_lazyfree_fail, I do not understand why it is still dummy_stat.
> If you want, I will keep dummy_stat, though.

My bad, I just misread this. 'stat' makes more sense then.