Messages in this thread | | | Subject | Re: [PATCH] thp: use is_zero_pfn after pte_present check | From | Vlastimil Babka <> | Date | Mon, 12 Oct 2015 17:15:06 +0200 |
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On 10/12/2015 04:57 PM, 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. ;-)
I'd also think this should go -stable, and I haven't heard the "real workload" argument before.
> If other reviewers are not against, I will Cc -stable in next spin. > >> >>> Signed-off-by: Minchan Kim <minchan@kernel.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@kernel.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@kernel.org>
So this patch should be stable 4.1+. Does it apply both in -next and 4.3-rcX?
> --- > 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; >
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