[PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free memory pattern

[Joel Fernandes (Google)]

In kfree_rcu() headless implementation (where the caller need not pass
an rcu_head, but rather directly pass a pointer to an object), we have
a fall-back where we allocate a rcu_head wrapper for the caller (not the
common case). This brings the pattern of needing to allocate some memory
to free some memory.  Currently we use GFP_ATOMIC flag to try harder for
this allocation, however the GFP_MEMALLOC flag is more tailored to this
pattern. We need to try harder not only during atomic context, but also
during non-atomic context anyway. So use the GFP_MEMALLOC flag instead.

Also remove the __GFP_NOWARN flag simply because although we do have a
synchronize_rcu() fallback for absolutely worst case, we still would
like to not enter that path and atleast trigger a warning to the user.

Cc: linux-mm@xxxxxxxxx
Cc: rcu@xxxxxxxxxxxxxxx
Cc: willy@xxxxxxxxxxxxx
Cc: peterz@xxxxxxxxxxxxx
Cc: neilb@xxxxxxxx
Cc: vbabka@xxxxxxx
Cc: mgorman@xxxxxxx
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---

This patch is based on the (not yet upstream) code in:
git://git.kernel.org/pub/scm/linux/kernel/git/jfern/linux.git (branch rcu/kfree)

It is a follow-up to the posted series:
https://lore.kernel.org/lkml/20200330023248.164994-1-joel@xxxxxxxxxxxxxxxxx/


 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 4be763355c9fb..965deefffdd58 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3149,7 +3149,7 @@ static inline struct rcu_head *attach_rcu_head_to_object(void *obj)
 
 	if (!ptr)
 		ptr = kmalloc(sizeof(unsigned long *) +
-				sizeof(struct rcu_head), GFP_ATOMIC | __GFP_NOWARN);
+				sizeof(struct rcu_head), GFP_MEMALLOC);
 
 	if (!ptr)
 		return NULL;
-- 
2.26.0.rc2.310.g2932bb562d-goog