On Tue, Mar 31, 2020 at 09:16:28AM -0400, Joel Fernandes (Google) wrote: > 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); Just to add, the main requirements here are: 1. Allocation should be bounded in time. 2. Allocation should try hard (possibly tapping into reserves) 3. Sleeping is Ok but should not affect the time bound. Considering this, GFP_MEMALLOC|GFP_NOWAIT seems appropriate. Thoughts? thanks, - Joel
