x86/mm: Fix boot crash caused by incorrect loop count calculation in sync_global_pgds()

Jeff Moyer reported that on his system with two memory regions 0~64G and
1T~1T+192G, and kernel option "memmap=192G!1024G" added, enabling KASLR
will make the system hang intermittently during boot. While adding 'nokaslr'
won't.

The back trace is:

 Oops: 0000 [#1] SMP

 RIP: memcpy_erms()
 [ .... ]
 Call Trace:
  pmem_rw_page()
  bdev_read_page()
  do_mpage_readpage()
  mpage_readpages()
  blkdev_readpages()
  __do_page_cache_readahead()
  force_page_cache_readahead()
  page_cache_sync_readahead()
  generic_file_read_iter()
  blkdev_read_iter()
  __vfs_read()
  vfs_read()
  SyS_read()
  entry_SYSCALL_64_fastpath()

This crash happens because the for loop count calculation in sync_global_pgds()
is not correct. When a mapping area crosses PGD entries, we should
calculate the starting address of region which next PGD covers and assign
it to next for loop count, but not add PGDIR_SIZE directly. The old
code works right only if the mapping area is an exact multiple of PGDIR_SIZE,
otherwize the end region could be skipped so that it can't be synchronized
to all other processes from kernel PGD init_mm.pgd.

In Jeff's system, emulated pmem area [1024G, 1216G) is smaller than
PGDIR_SIZE. While 'nokaslr' works because PAGE_OFFSET is 1T aligned, it
makes this area be mapped inside one PGD entry. With KASLR enabled,
this area could cross two PGD entries, then the next PGD entry won't
be synced to all other processes. That is why we saw empty PGD.

Fix it.

Reported-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Baoquan He <bhe@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jinbum Park <jinb.park7@gmail.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Garnier <thgarnie@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Yasuaki Ishimatsu <yasu.isimatu@gmail.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1493864747-8506-1-git-send-email-bhe@redhat.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>