From: Jan Stancek <jstancek@redhat.com>
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
Subject: [PATCH v2] mm: page_mapped: don't assume compound page is huge or THP
Date: Fri, 30 Nov 2018 13:06:57 +0100 [thread overview]
Message-ID: <c440d69879e34209feba21e12d236d06bc0a25db.1543577156.git.jstancek@redhat.com> (raw)
In-Reply-To: <eabca57aa14f4df723173b24891f4a2d9c501f21.1543526537.git.jstancek@redhat.com>
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
(for HPAGE_PMD_NR iterations) 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
Fix the loop to iterate for "1 << compound_order" pages.
Debugged-by: Laszlo Ersek <lersek@redhat.com>
Suggested-by: "Kirill A. Shutemov" <kirill@shutemov.name>
Signed-off-by: Jan Stancek <jstancek@redhat.com>
---
mm/util.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
Changes in v2:
- change the loop instead so we check also mapcount of subpages
diff --git a/mm/util.c b/mm/util.c
index 8bf08b5b5760..5c9c7359ee8a 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -478,7 +478,7 @@ bool page_mapped(struct page *page)
return true;
if (PageHuge(page))
return false;
- for (i = 0; i < hpage_nr_pages(page); i++) {
+ for (i = 0; i < (1 << compound_order(page)); i++) {
if (atomic_read(&page[i]._mapcount) >= 0)
return true;
}
--
1.8.3.1
next prev parent reply other threads:[~2018-11-30 12:07 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-29 21:53 [PATCH] mm: page_mapped: don't assume compound page is huge or THP Jan Stancek
2018-11-30 10:19 ` Kirill A. Shutemov
2018-11-30 12:06 ` Jan Stancek [this message]
2018-11-30 12:18 ` [PATCH v2] " Michal Hocko
2018-11-30 12:36 ` Kirill A. Shutemov
2018-11-30 12:45 ` Michal Hocko
2018-11-30 12:55 ` Kirill A. Shutemov
2018-11-30 15:53 ` David Hildenbrand
2018-12-03 10:23 ` Laszlo Ersek
2018-12-03 11:28 ` Kirill A. Shutemov
2019-01-04 21:56 ` Andrea Arcangeli
2019-02-04 14:38 ` Lars Persson
2019-02-05 7:14 ` Jan Stancek
2019-02-18 13:43 ` Lars Persson
2019-02-18 13:50 ` Michal Hocko
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