linux-kernel - Re: [PATCH 0/5] MADV_FREE refactoring and fix KSM page

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Date:	Tue, 20 Oct 2015 16:21:09 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Michal Hocko <mhocko@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>,
	Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH 0/5] MADV_FREE refactoring and fix KSM page

On Mon, Oct 19, 2015 at 07:01:50PM +0900, Minchan Kim wrote:
> On Mon, Oct 19, 2015 at 03:31:42PM +0900, Minchan Kim wrote:
> > Hello, it's too late since I sent previos patch.
> > https://lkml.org/lkml/2015/6/3/37
> > 
> > This patch is alomost new compared to previos approach.
> > I think this is more simple, clear and easy to review.
> > 
> > One thing I should notice is that I have tested this patch
> > and couldn't find any critical problem so I rebased patchset
> > onto recent mmotm(ie, mmotm-2015-10-15-15-20) to send formal
> > patchset. Unfortunately, I start to see sudden discarding of
> > the page we shouldn't do. IOW, application's valid anonymous page
> > was disappeared suddenly.
> > 
> > When I look through THP changes, I think we could lose
> > dirty bit of pte between freeze_page and unfreeze_page
> > when we mark it as migration entry and restore it.
> > So, I added below simple code without enough considering
> > and cannot see the problem any more.
> > I hope it's good hint to find right fix this problem.
> > 
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index d5ea516ffb54..e881c04f5950 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -3138,6 +3138,9 @@ static void unfreeze_page_vma(struct vm_area_struct *vma, struct page *page,
> >  		if (is_write_migration_entry(swp_entry))
> >  			entry = maybe_mkwrite(entry, vma);
> >  
> > +		if (PageDirty(page))
> > +			SetPageDirty(page);
> 
> The condition of PageDirty was typo. I didn't add the condition.
> Just added.
> 
>                 SetPageDirty(page);

I reviewed THP refcount redesign patch and It seems below patch fixes
MADV_FREE problem. It works well for hours.

>From 104a0940b4c0f97e61de9fee0fd602926ff28312 Mon Sep 17 00:00:00 2001
From: Minchan Kim <minchan@...nel.org>
Date: Tue, 20 Oct 2015 16:00:52 +0900
Subject: [PATCH] mm: mark head page dirty in split_huge_page

In thp split in old THP refcount, we mappped all of pages
(ie, head + tails) to pte_mkdirty and mark PG_flags to every
tail pages.

But with THP refcount redesign, we can lose dirty bit in page table
and PG_dirty for head page if we want to free the THP page using
migration_entry.

It ends up discarding head page by madvise_free suddenly.
This patch fixes it by mark the head page PG_dirty when VM splits
the THP page.

Signed-off-by: Minchan Kim <minchan@...nel.org>
---
 mm/huge_memory.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index adccfb48ce57..7fbbd42554a1 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3258,6 +3258,7 @@ static void __split_huge_page(struct page *page, struct list_head *list)
 	atomic_sub(tail_mapcount, &head->_count);
 
 	ClearPageCompound(head);
+	SetPageDirty(head);
 	spin_unlock_irq(&zone->lru_lock);
 
 	unfreeze_page(page_anon_vma(head), head);
-- 
1.9.1

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