[PATCH] mm: allow page_cache_get_speculative in interrupt context

[kan . liang]

From: Kan Liang <kan.liang@xxxxxxxxx>

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

The bug was introduced by commit:

  2947ba054a4d ("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().

Signed-off-by: Kan Liang <kan.liang@xxxxxxxxx>
---
 include/linux/pagemap.h | 2 --
 1 file changed, 2 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index baa9344..79b36f5 100644
--- a/include/linux/pagemap.h
+++ b/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());
-- 
2.4.3