mm: fix data race in PagePoisoned()

PagePoisoned() accesses page->flags which can be updated concurrently:

  | BUG: KCSAN: data-race in next_uptodate_page / unlock_page
  |
  | write (marked) to 0xffffea00050f37c0 of 8 bytes by task 1872 on cpu 1:
  |  instrument_atomic_write           include/linux/instrumented.h:87 [inline]
  |  clear_bit_unlock_is_negative_byte include/asm-generic/bitops/instrumented-lock.h:74 [inline]
  |  unlock_page+0x102/0x1b0           mm/filemap.c:1465
  |  filemap_map_pages+0x6c6/0x890     mm/filemap.c:3057
  |  ...
  | read to 0xffffea00050f37c0 of 8 bytes by task 1873 on cpu 0:
  |  PagePoisoned                   include/linux/page-flags.h:204 [inline]
  |  PageReadahead                  include/linux/page-flags.h:382 [inline]
  |  next_uptodate_page+0x456/0x830 mm/filemap.c:2975
  |  ...
  | CPU: 0 PID: 1873 Comm: systemd-udevd Not tainted 5.11.0-rc4-00001-gf9ce0be71d1f #1

To avoid the compiler tearing or otherwise optimizing the access, use
READ_ONCE() to access flags.

Link: https://lore.kernel.org/all/20210826144157.GA26950@xsang-OptiPlex-9020/
Link: https://lkml.kernel.org/r/20210913113542.2658064-1-elver@google.com
Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Marco Elver <elver@google.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Will Deacon <will@kernel.org>
Cc: Marco Elver <elver@google.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>

include/linux/page-flags.h

@@ -206,7 +206,7 @@ static __always_inline int PageCompound(struct page *page)
 #define	PAGE_POISON_PATTERN	-1l
 static inline int PagePoisoned(const struct page *page)
 {
-	return page->flags == PAGE_POISON_PATTERN;
+	return READ_ONCE(page->flags) == PAGE_POISON_PATTERN;
 }
 
 #ifdef CONFIG_DEBUG_VM