From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lf0-f71.google.com (mail-lf0-f71.google.com [209.85.215.71]) by kanga.kvack.org (Postfix) with ESMTP id 9EB816B007E for ; Thu, 2 Jun 2016 11:51:54 -0400 (EDT) Received: by mail-lf0-f71.google.com with SMTP id o70so26117195lfg.1 for ; Thu, 02 Jun 2016 08:51:54 -0700 (PDT) Received: from mail-lf0-x244.google.com (mail-lf0-x244.google.com. [2a00:1450:4010:c07::244]) by mx.google.com with ESMTPS id 99si540153lfr.277.2016.06.02.08.51.52 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 02 Jun 2016 08:51:53 -0700 (PDT) Received: by mail-lf0-x244.google.com with SMTP id 65so5546284lfq.1 for ; Thu, 02 Jun 2016 08:51:52 -0700 (PDT) Date: Thu, 2 Jun 2016 18:51:50 +0300 From: "Kirill A. Shutemov" Subject: Re: [BUG/REGRESSION] THP: broken page count after commit aa88b68c Message-ID: <20160602155149.GB8493@node.shutemov.name> References: <20160602172141.75c006a9@thinkpad> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160602172141.75c006a9@thinkpad> Sender: owner-linux-mm@kvack.org List-ID: To: Gerald Schaefer Cc: "Kirill A. Shutemov" , Andrea Arcangeli , "Aneesh Kumar K.V" , Mel Gorman , Hugh Dickins , Johannes Weiner , Dave Hansen , Vlastimil Babka , Andrew Morton , Linus Torvalds , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Christian Borntraeger , Martin Schwidefsky , Heiko Carstens On Thu, Jun 02, 2016 at 05:21:41PM +0200, Gerald Schaefer wrote: > Christian Borntraeger reported a kernel panic after corrupt page counts, > and it turned out to be a regression introduced with commit aa88b68c > "thp: keep huge zero page pinned until tlb flush", at least on s390. > > put_huge_zero_page() was moved over from zap_huge_pmd() to release_pages(), > and it was replaced by tlb_remove_page(). However, release_pages() might > not always be triggered by (the arch-specific) tlb_remove_page(). > > On s390 we call free_page_and_swap_cache() from tlb_remove_page(), and not > tlb_flush_mmu() -> free_pages_and_swap_cache() like the generic version, > because we don't use the MMU-gather logic. Although both functions have very > similar names, they are doing very unsimilar things, in particular > free_page_xxx is just doing a put_page(), while free_pages_xxx calls > release_pages(). > > This of course results in very harmful put_page()s on the huge zero page, > on architectures where tlb_remove_page() is implemented in this way. It > seems to affect only s390 and sh, but sh doesn't have THP support, so > the problem (currently) probably only exists on s390. > > The following quick hack fixed the issue: > > diff --git a/mm/swap_state.c b/mm/swap_state.c > index 0d457e7..c99463a 100644 > --- a/mm/swap_state.c > +++ b/mm/swap_state.c > @@ -252,7 +252,10 @@ static inline void free_swap_cache(struct page *page) > void free_page_and_swap_cache(struct page *page) > { > free_swap_cache(page); > - put_page(page); > + if (is_huge_zero_page(page)) > + put_huge_zero_page(); > + else > + put_page(page); > } > > /* The fix looks good to me. > But of course there might be a better solution, and there still are some > questions left: > - Why does free_page_xxx() behave so differently from free_pages_xxx()? I don't see it behave too deiferently. It just try to batch freeing to lower locking overhead. > - Would it be OK to implement free_page_xxx() by calling free_pages_xxx() > with nr = 1, similar to free_page() vs. free_pages()? > - Would it be OK to replace the put_page() in free_page_xxx() with a call > to release_pages() with nr = 1? release_pages() somewhat suboptimal for nr=1. I guess we can fix this with shortcut to put_page() at start of release_page() if nr == 1. > - Would it be better to fix this in the arch-specific tlb_remove_page(), > by calling free_pages_xxx() with nr = 1 instead of free_page_xxx()? > > Regards, > Gerald > > -- > 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 -- 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