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

The 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-g60b54823) 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.

Kirrill said "IIRC, sound subsystem can producuce custom mapped compound
pages".

Link: http://lkml.kernel.org/r/c440d69879e34209feba21e12d236d06bc0a25db.1543577156.git.jstancek@redhat.com
Fixes: e1534ae9 ("mm: differentiate page_mapped() from page_mapcount() for compound pages")
Signed-off-by: Jan Stancek <jstancek@redhat.com>
Debugged-by: Laszlo Ersek <lersek@redhat.com>
Suggested-by: "Kirill A. Shutemov" <kirill@shutemov.name>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>