[PATCH v2 2/3] uprobes: change build_map_info() to try kmalloc(GFP_NOWAIT) first

[Oleg Nesterov <oleg@redhat.com>]

build_map_info() doesn't allocate the memory under i_mmap_mutex
to avoid the deadlock with page reclaim. But it can try GFP_NOWAIT
first, it should work in the likely case and thus we almost never
need the pre-alloc-and-retry path.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/events/uprobes.c |   10 ++++++++++
 1 files changed, 10 insertions(+), 0 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index a1bbcab..88e903d 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -771,6 +771,16 @@ build_map_info(struct address_space *mapping, loff_t offset, bool is_register)
 		if (!valid_vma(vma, is_register))
 			continue;
 
+		if (!prev && !more) {
+			/*
+			 * Needs GFP_NOWAIT to avoid i_mmap_mutex recursion through
+			 * reclaim. This is optimistic, no harm done if it fails.
+			 */
+			prev = kmalloc(sizeof(struct map_info),
+					GFP_NOWAIT | __GFP_NOMEMALLOC | __GFP_NOWARN);
+			if (prev)
+				prev->next = NULL;
+		}
 		if (!prev) {
 			more++;
 			continue;
-- 
1.5.5.1