Message-ID: <20151012145746.GA11396@bbox>
Date:	Mon, 12 Oct 2015 23:57:46 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] thp: use is_zero_pfn after pte_present check

Hello,

On Mon, Oct 12, 2015 at 01:13:20PM +0300, Kirill A. Shutemov wrote:
> On Mon, Oct 12, 2015 at 10:54:16AM +0900, Minchan Kim wrote:
> > Use is_zero_pfn on pteval only after pte_present check on pteval
> > (It might be better idea to introduce is_zero_pte where checks
> > pte_present first). Otherwise, it could work with swap or
> > migration entry and if pte_pfn's result is equal to zero_pfn
> > by chance, we lose user's data in __collapse_huge_page_copy.
> > So if you're luck, the application is segfaulted and finally you
> > could see below message when the application is exit.
> > 
> > BUG: Bad rss-counter state mm:ffff88007f099300 idx:2 val:3
> 
> Did you acctually steped on the bug?
> If yes it's subject for stable@, I think.

Yes, I did with my testing program which made heavy swap-in/out/
swapoff with MADV_DONTNEED in a memcg.
Actually, I marked this patch as -stable but removed it right before
sending because my test program is artificial and didn't see any
report about rss bad counting with MM_SWAPENTS in linux-mm(Of course,
I might miss it).
In addition, sometime I saw someone insists on "It's not a stable
material if it's not a bug with real workload". I don't want to
involve such non-technical stuff so waited someone nudges me to
mark it as -stable and finally, you did. ;-)
If other reviewers are not against, I will Cc -stable in next spin.

> 
> > Signed-off-by: Minchan Kim <minchan@...nel.org>
> > ---
> > 
> > I found this bug with MADV_FREE hard test. Sometime, I saw
> > "Bad rss-counter" message with MM_SWAPENTS but it's really
> > rare, once a day if I was luck or once in five days if I was
> > unlucky so I am doing test still and just pass a few days but
> > I hope it will fix the issue.
> > 
> >  mm/huge_memory.c | 12 +++++++++++-
> >  1 file changed, 11 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 4b06b8db9df2..349590aa4533 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2665,15 +2665,25 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> >  	for (_address = address, _pte = pte; _pte < pte+HPAGE_PMD_NR;
> >  	     _pte++, _address += PAGE_SIZE) {
> >  		pte_t pteval = *_pte;
> > -		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> > +		if (pte_none(pteval)) {
> 
> In -mm tree we have is_swap_pte() check before this point in
> khugepaged_scan_pmd()

Actually, I tested this patch with v4.2 kernel so it doesn't have
the check.
Now, I look through optimistic check for swapin readahead patch
in current mmotm.
It seems the check couldn't prevent this problem because it releases
pte lock and anon_vma lock before being isolated the page in
__collapse_huge_page_isolate so the page could be swapped out again.

> 
> Also, what about similar pattern in __collapse_huge_page_isolate() and
> __collapse_huge_page_copy()? Shouldn't they be fixed as well?

I see what's wrong here.
/me slaps self.
The line I was about to change was in __collapse_huge_page_isolate
but I changed khugepaged_scan_pmd by mistake at last modification
since that part is almost same. :(
Fortunately my testing kernel is doing right version.
Here it goes.

>From 2a2e4b247e132d823af30655dbc0b57738e9d6ee Mon Sep 17 00:00:00 2001
From: Minchan Kim <minchan@...nel.org>
Date: Mon, 12 Oct 2015 09:52:46 +0900
Subject: [PATCH] thp: use is_zero_pfn only after pte_present check

Use is_zero_pfn on pteval only after pte_present check on pteval
(It might be better idea to introduce is_zero_pte where checks
pte_present first). Otherwise, it could work with swap or
migration entry and if pte_pfn's result is equal to zero_pfn
by chance, we lose user's data in __collapse_huge_page_copy.
So if you're luck, the application is segfaulted and finally you
could see below message when the application is exit.

BUG: Bad rss-counter state mm:ffff88007f099300 idx:2 val:3

Signed-off-by: Minchan Kim <minchan@...nel.org>
---
 mm/huge_memory.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 4b06b8db9df2..bbac913f96bc 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2206,7 +2206,8 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 	for (_pte = pte; _pte < pte+HPAGE_PMD_NR;
 	     _pte++, address += PAGE_SIZE) {
 		pte_t pteval = *_pte;
-		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
+		if (pte_none(pteval) || (pte_present(pteval) &&
+				is_zero_pfn(pte_pfn(pteval)))) {
 			if (!userfaultfd_armed(vma) &&
 			    ++none_or_zero <= khugepaged_max_ptes_none)
 				continue;
-- 
1.9.1


In khugepaged_scan_pmd, although there is no is_swap_pte check in
v4.2, we don't need to check pte_present check right before is_zero_pfn
because that part is just scanning operation so even if something wrong
happens rarely, it should filter out in __collapse_huge_page_isolate
with this patch.

In __collapse_huge_page_copy, we don't need the check, either.
Because every ptes in the vma's 2M area point out isolated LRU pages
and zero page so any pages couldn't be swap-out.

Thanks for the review.
--
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