[PATCH] mm: do not rely on preempt_count in print_vma_addr

[Michal Hocko]

The preempt count check on print_vma_addr has been added by e8bff74afbdb
("x86: fix "BUG: sleeping function called from invalid context" in
print_vma_addr()") and it relied on the elevated preempt count from
preempt_conditional_sti because preempt_count check doesn't work on
non preemptive kernels by default. The code has evolved though and
d99e1bd175f4 ("x86/entry/traps: Refactor preemption and interrupt flag
handling") has replaced preempt_conditional_sti by an explicit
preempt_disable which is noop on !PREEMPT so the check in print_vma_addr
is broken.

Fix the issue by using trylock on mmap_sem rather than chacking the
preempt count. The allocation we are relying on has to be GFP_NOWAIT
as well. There is a chance that we won't dump the vma state if the lock
is contended or the memory short but this is acceptable outcome and much
less fragile than the not working preemption check or tricks around it.

Fixes: d99e1bd175f4 ("x86/entry/traps: Refactor preemption and interrupt flag handling")
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 mm/memory.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index a728bed16c20..1e308ac8ca0a 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4457,17 +4457,15 @@ void print_vma_addr(char *prefix, unsigned long ip)
 	struct vm_area_struct *vma;
 
 	/*
-	 * Do not print if we are in atomic
-	 * contexts (in exception stacks, etc.):
+	 * we might be running from an atomic context so we cannot sleep
 	 */
-	if (preempt_count())
+	if (!down_read_trylock(&mm->mmap_sem))
 		return;
 
-	down_read(&mm->mmap_sem);
 	vma = find_vma(mm, ip);
 	if (vma && vma->vm_file) {
 		struct file *f = vma->vm_file;
-		char *buf = (char *)__get_free_page(GFP_KERNEL);
+		char *buf = (char *)__get_free_page(GFP_NOWAIT);
 		if (buf) {
 			char *p;
 
-- 
2.14.2

-- 
Michal Hocko
SUSE Labs