From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt1-f198.google.com (mail-qt1-f198.google.com [209.85.160.198]) by kanga.kvack.org (Postfix) with ESMTP id 535A96B54C6 for ; Thu, 29 Nov 2018 16:55:25 -0500 (EST) Received: by mail-qt1-f198.google.com with SMTP id p24so3394827qtl.2 for ; Thu, 29 Nov 2018 13:55:25 -0800 (PST) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id s17si1809175qve.22.2018.11.29.13.55.24 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 29 Nov 2018 13:55:24 -0800 (PST) From: Jan Stancek Subject: [PATCH] mm: page_mapped: don't assume compound page is huge or THP Date: Thu, 29 Nov 2018 22:53:48 +0100 Message-Id: Sender: owner-linux-mm@kvack.org List-ID: To: linux-mm@kvack.org, lersek@redhat.com, alex.williamson@redhat.com, aarcange@redhat.com, rientjes@google.com, kirill@shutemov.name, mgorman@techsingularity.net, mhocko@suse.com, jstancek@redhat.com Cc: linux-kernel@vger.kernel.org LTP proc01 testcase has been observed to rarely trigger crashes on arm64: page_mapped+0x78/0xb4 stable_page_flags+0x27c/0x338 kpageflags_read+0xfc/0x164 proc_reg_read+0x7c/0xb8 __vfs_read+0x58/0x178 vfs_read+0x90/0x14c SyS_read+0x60/0xc0 Issue is that page_mapped() assumes that if compound page is not huge, then it must be THP. But if this is 'normal' compound page (COMPOUND_PAGE_DTOR), then following loop can keep running until it tries to read from memory that isn't mapped and triggers a panic: for (i = 0; i < hpage_nr_pages(page); i++) { if (atomic_read(&page[i]._mapcount) >= 0) return true; } I could replicate this on x86 (v4.20-rc4-98-g60b548237fed) only with a custom kernel module [1] which: - allocates compound page (PAGEC) of order 1 - allocates 2 normal pages (COPY), which are initialized to 0xff (to satisfy _mapcount >= 0) - 2 PAGEC page structs are copied to address of first COPY page - second page of COPY is marked as not present - call to page_mapped(COPY) now triggers fault on access to 2nd COPY page at offset 0x30 (_mapcount) [1] https://github.com/jstancek/reproducers/blob/master/kernel/page_mapped_crash/repro.c This patch modifies page_mapped() to check for 'normal' compound pages (COMPOUND_PAGE_DTOR). Debugged-by: Laszlo Ersek Signed-off-by: Jan Stancek --- include/linux/mm.h | 9 +++++++++ mm/util.c | 2 ++ 2 files changed, 11 insertions(+) diff --git a/include/linux/mm.h b/include/linux/mm.h index 5411de93a363..18b0bb953f92 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -700,6 +700,15 @@ static inline compound_page_dtor *get_compound_page_dtor(struct page *page) return compound_page_dtors[page[1].compound_dtor]; } +static inline int PageNormalCompound(struct page *page) +{ + if (!PageCompound(page)) + return 0; + + page = compound_head(page); + return page[1].compound_dtor == COMPOUND_PAGE_DTOR; +} + static inline unsigned int compound_order(struct page *page) { if (!PageHead(page)) diff --git a/mm/util.c b/mm/util.c index 8bf08b5b5760..06c1640cb7b3 100644 --- a/mm/util.c +++ b/mm/util.c @@ -478,6 +478,8 @@ bool page_mapped(struct page *page) return true; if (PageHuge(page)) return false; + if (PageNormalCompound(page)) + return false; for (i = 0; i < hpage_nr_pages(page); i++) { if (atomic_read(&page[i]._mapcount) >= 0) return true; -- 1.8.3.1