mm: fix devm_memremap_pages crash, use mem_hotplug_{begin, done}

BugLink: http://bugs.launchpad.net/bugs/1658091

commit f931ab4 upstream.

Both arch_add_memory() and arch_remove_memory() expect a single threaded
context.

For example, arch/x86/mm/init_64.c::kernel_physical_mapping_init() does
not hold any locks over this check and branch:

    if (pgd_val(*pgd)) {
    	pud = (pud_t *)pgd_page_vaddr(*pgd);
    	paddr_last = phys_pud_init(pud, __pa(vaddr),
    				   __pa(vaddr_end),
    				   page_size_mask);
    	continue;
    }

    pud = alloc_low_page();
    paddr_last = phys_pud_init(pud, __pa(vaddr), __pa(vaddr_end),
    			   page_size_mask);

The result is that two threads calling devm_memremap_pages()
simultaneously can end up colliding on pgd initialization.  This leads
to crash signatures like the following where the loser of the race
initializes the wrong pgd entry:

    BUG: unable to handle kernel paging request at ffff888ebfff0000
    IP: memcpy_erms+0x6/0x10
    PGD 2f8e8fc067 PUD 0 /* <---- Invalid PUD */
    Oops: 0000 [#1] SMP DEBUG_PAGEALLOC
    CPU: 54 PID: 3818 Comm: systemd-udevd Not tainted 4.6.7+ #13
    task: ffff882fac290040 ti: ffff882f887a4000 task.ti: ffff882f887a4000
    RIP: memcpy_erms+0x6/0x10
    [..]
    Call Trace:
      ? pmem_do_bvec+0x205/0x370 [nd_pmem]
      ? blk_queue_enter+0x3a/0x280
      pmem_rw_page+0x38/0x80 [nd_pmem]
      bdev_read_page+0x84/0xb0

Hold the standard memory hotplug mutex over calls to
arch_{add,remove}_memory().

Fixes: 41e94a8 ("add devm_memremap_pages")
Link: http://lkml.kernel.org/r/148357647831.9498.12606007370121652979.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

kernel/memremap.c

@@ -159,7 +159,9 @@ static void devm_memremap_pages_release(struct device *dev, void *res)
 	struct page_map *page_map = res;
 
 	/* pages are dead and unused, undo the arch mapping */
+	mem_hotplug_begin();
 	arch_remove_memory(page_map->res.start, resource_size(&page_map->res));
+	mem_hotplug_done();
 }
 
 void *devm_memremap_pages(struct device *dev, struct resource *res)
@@ -189,7 +191,9 @@ void *devm_memremap_pages(struct device *dev, struct resource *res)
 	if (nid < 0)
 		nid = numa_mem_id();
 
+	mem_hotplug_begin();
 	error = arch_add_memory(nid, res->start, resource_size(res), true);
+	mem_hotplug_done();
 	if (error) {
 		devres_free(page_map);
 		return ERR_PTR(error);