linux-kernel - Re: [patch 5/5]thp: split huge page if head page is isolated

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Date:	Thu, 10 Nov 2011 11:23:06 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"aarcange@...hat.com" <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>, mel <mel@....ul.ie>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm <linux-mm@...ck.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [patch 5/5]thp: split huge page if head page is isolated

So long contents.
Let's remove it.

On Thu, Nov 10, 2011 at 10:07:10AM +0800, Shaohua Li wrote:

<snip>

> > > Coudn't we make both sides good?
> > > 
> > > Here is my quick patch.
> > > How about this?
> > > It doesn't split THPs in page_list but still reclaims non-THPs so
> > > I think it doesn't changed old behavior a lot.
> > I like this idea, will do some test soon.
> hmm, this doesn't work as expected. The putback_lru_page() messes lru.
> This isn't a problem if the page will be written since
> rotate_reclaimable_page() will fix the order. I got worse data than my
> v2 patch, eg, more thp_fallbacks, mess lru order, more pages are
> scanned. We could add something like putback_lru_page_tail, but I'm not

Hmm, It's not LRU mess problem. but it's just guessing and you might be right
because you have a workload and can test it.

My guessing is that cull_mlocked reset synchronus page reclaim.
Could you test this patch, again?

And, if the problem cause by LRU mess, I think it is valuable with adding putback_lru_page_tail
because thp added lru_add_page_tail, too.

Thanks!

diff --git a/mm/vmscan.c b/mm/vmscan.c
index b55699c..e2c84c2 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -767,6 +767,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 	unsigned long nr_dirty = 0;
 	unsigned long nr_congested = 0;
 	unsigned long nr_reclaimed = 0;
+	bool split_thp = false;
+	bool swapout_thp = false;
 
 	cond_resched();
 
@@ -784,6 +786,14 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 		if (!trylock_page(page))
 			goto keep;
 
+		/*
+		 * If we already swap out a THP, we don't want to
+		 * split THPs any more. Let's wait until dirty a thp page
+		 * to be written into swap device
+		 */
+		if (unlikely(swapout_thp && PageTransHuge(page)))
+			goto pass_thp;
+
 		VM_BUG_ON(PageActive(page));
 		VM_BUG_ON(page_zone(page) != zone);
 
@@ -838,6 +848,12 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 		if (PageAnon(page) && !PageSwapCache(page)) {
 			if (!(sc->gfp_mask & __GFP_IO))
 				goto keep_locked;
+			if (unlikely(PageTransHuge(page)))
+				if (unlikely(split_huge_page_list(page,
+					page_list)))
+				    goto activate_locked;
+				else
+					split_thp = true;
 			if (!add_to_swap(page))
 				goto activate_locked;
 			may_enter_fs = 1;
@@ -880,6 +896,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 			case PAGE_ACTIVATE:
 				goto activate_locked;
 			case PAGE_SUCCESS:
+				if (split_thp)
+					swapout_thp = true;
 				if (PageWriteback(page))
 					goto keep_lumpy;
 				if (PageDirty(page))
@@ -962,6 +980,10 @@ free_it:
 		list_add(&page->lru, &free_pages);
 		continue;
 
+pass_thp:
+		unlock_page(page);
+		putback_lru_page(page);
+		continue;
 cull_mlocked:
 		if (PageSwapCache(page))
 			try_to_free_swap(page);

> convinced it's worthy(even with it, we still will mess lru a little). So
> I'm back to use the v2 patch if no better solution, it's still much
> better than current code.
> 
> Thanks,
> Shaohua
> 

-- 
Kind regards,
Minchan Kim
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