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

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Message-ID: <20200423200020.GB46847@google.com>
Date:   Thu, 23 Apr 2020 13:00:20 -0700
From:   Minchan Kim <minchan@...nel.org>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Jaewon Kim <jaewon31.kim@...sung.com>, 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 12:05:46PM -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.