[PATCH v2] mm: page_mapped: don't assume compound page is huge or THP

[Jan Stancek]

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@xxxxxxxxxx>
Suggested-by: "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx>
Signed-off-by: Jan Stancek <jstancek@xxxxxxxxxx>
---
 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