mm: allow page_cache_get_speculative in interrupt context

Kernel panic when calling the IRQ-safe __get_user_pages_fast in NMI
handler.

The bug was introduced by commit 2947ba0 ("x86/mm/gup: Switch GUP
to the generic get_user_page_fast() implementation").

The original x86 __get_user_page_fast used plain get_page() or
page_ref_add().  However, the generic __get_user_page_fast uses
page_cache_get_speculative(), which has VM_BUG_ON(in_interrupt()).

There is no reason to prevent page_cache_get_speculative from using in
interrupt context.  According to the author, putting a BUG_ON there is
just because the code is not verifying correctness of interrupt races.
I did some tests in interrupt context.  There is no issue found.

Removing VM_BUG_ON(in_interrupt()) for page_cache_get_speculative().

Link: http://lkml.kernel.org/r/[email protected]
Fixes: 2947ba0 ("x86/mm/gup: Switch GUP to the generic get_user_page_fast() implementation")
Signed-off-by: Kan Liang <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Ying Huang <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Ingo Molnar <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

include/linux/pagemap.h

@@ -163,8 +163,6 @@ void release_pages(struct page **pages, int nr, bool cold);
  */
 static inline int page_cache_get_speculative(struct page *page)
 {
-	VM_BUG_ON(in_interrupt());
-
 #ifdef CONFIG_TINY_RCU
 # ifdef CONFIG_PREEMPT_COUNT
 	VM_BUG_ON(!in_atomic() && !irqs_disabled());