From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wi0-f182.google.com (mail-wi0-f182.google.com [209.85.212.182]) by kanga.kvack.org (Postfix) with ESMTP id 0AF796B0253 for ; Mon, 12 Oct 2015 11:27:36 -0400 (EDT) Received: by wicgb1 with SMTP id gb1so54880039wic.1 for ; Mon, 12 Oct 2015 08:27:35 -0700 (PDT) Received: from mail-wi0-f176.google.com (mail-wi0-f176.google.com. [209.85.212.176]) by mx.google.com with ESMTPS id df4si20337504wjb.149.2015.10.12.08.27.34 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 12 Oct 2015 08:27:34 -0700 (PDT) Received: by wicge5 with SMTP id ge5so22382238wic.0 for ; Mon, 12 Oct 2015 08:27:34 -0700 (PDT) Date: Mon, 12 Oct 2015 18:27:33 +0300 From: "Kirill A. Shutemov" Subject: Re: [PATCH] thp: use is_zero_pfn after pte_present check Message-ID: <20151012152733.GA6447@node> References: <1444614856-18543-1-git-send-email-minchan@kernel.org> <20151012101320.GB2544@node> <20151012145746.GA11396@bbox> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151012145746.GA11396@bbox> Sender: owner-linux-mm@kvack.org List-ID: To: Minchan Kim Cc: Andrew Morton , Mel Gorman , Vlastimil Babka , Andrea Arcangeli , Hugh Dickins , Rik van Riel , linux-kernel@vger.kernel.org, linux-mm@kvack.org On Mon, Oct 12, 2015 at 11:57:46PM +0900, Minchan Kim wrote: > 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 > > > --- > > > > > > 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 > 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 Acked-by: Kirill A. Shutemov > --- > 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. -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org