Re: [PATCH tip/core/rcu 14/15] rcu/tree: Allocate a page when caller is preemptible

["Paul E. McKenney" <paulmck@kernel.org>]

On Thu, Oct 01, 2020 at 11:02:20AM +0200, Michal Hocko wrote:
> On Wed 30-09-20 16:21:54, Paul E. McKenney wrote:
> > On Wed, Sep 30, 2020 at 10:41:39AM +0200, Michal Hocko wrote:
> > > On Tue 29-09-20 18:53:27, Paul E. McKenney wrote:
> [...]
> > > > No argument on it being confusing, and I hope that the added header
> > > > comment helps.  But specifically, can_sleep==true is a promise by the
> > > > caller to be schedulable and not to be holding any lock/mutex/whatever
> > > > that might possibly be acquired by the memory allocator or by anything
> > > > else that the memory allocator might invoke, to your point, including
> > > > for but one example the reclaim logic.
> > > > 
> > > > The only way that can_sleep==true is if this function was invoked due
> > > > to a call to single-argument kvfree_rcu(), which must be schedulable
> > > > because its fallback is to invoke synchronize_rcu().
> > > 
> > > OK. I have to say that it is still not clear to me whether this call
> > > path can be called from the memory reclaim context. If yes then you need
> > > __GFP_NOMEMALLOC as well.
> > 
> > Right now the restriction is that single-argument (AKA can_sleep==true)
> > kvfree_rcu() cannot be invoked from memory reclaim context.
> > 
> > But would adding __GFP_NOMEMALLOC to the can_sleep==true GFP_ flags
> > allow us to remove this restriction?  If so, I will queue a separate
> > patch making this change.  The improved ease of use would be well
> > worth it, if I understand correctly (ha!!!).
> 
> It would be quite daring to claim it will be ok but it will certainly be
> less problematic. Adding the flag will not hurt in any case. As this is
> a shared called that might be called from many contexts I think it will
> be safer to have it there. The justification is that it will prevent
> consumption of memory reserves from MEMALLOC contexts.

Ah, so a different goal (and yes, I finally went over and read the
relevant documentation).  Agreed, the can_sleep path does not really
need to be dipping into the emergency reserves.  And it looks like the
not-from-reclaim restriction is still at least partially in effect,
but one step at a time.

The patch is shown below, which I have queued for a later release.

> > > [...]
> > > 
> > > > > What is the point of calling kmalloc  for a PAGE_SIZE object? Wouldn't
> > > > > using the page allocator directly be better?
> > > > 
> > > > Well, you guys gave me considerable heat about abusing internal allocator
> > > > interfaces, and kmalloc() and kfree() seem to be about as non-internal
> > > > as you can get and still be invoking the allocator.  ;-)
> > > 
> > > alloc_pages resp. __get_free_pages is a normal page allocator interface
> > > to use for page size granular allocations. kmalloc is for more fine
> > > grained allocations.
> > 
> > OK, in the short term, both work, but I have queued a separate patch
> > making this change and recording the tradeoffs.  This is not yet a
> > promise to push this patch, but it is a promise not to lose this part
> > of the picture.  Please see below.
> 
> It doesn't matter all that much. Both allocators will work. It is just a
> matter of using optimal tool for the specific purose.
> 
> > You mentioned alloc_pages().  I reverted to __get_free_pages(), but
> > alloc_pages() of course looks nicer.  What are the tradeoffs between
> > __get_free_pages() and alloc_pages()?
> 
> alloc_pages will return struct page but you need a kernel pointer. That
> is what __get_free_pages will give you (or you can call page_address
> directly).

Thank you, looks like __get_free_pages() is the tool for this job.

Please see below for the aforementioned patch.

							Thanx, Paul

------------------------------------------------------------------------

commit ea5c19d21233b5e8d3d06c0d4ecd6be9f2829dc3
Author: Paul E. McKenney <paulmck@kernel.org>
Date:   Thu Oct 1 09:24:40 2020 -0700

    kvfree_rcu: Use __GFP_NOMEMALLOC for single-argument kvfree_rcu()
    
    This commit applies the __GFP_NOMEMALLOC gfp flag to memory allocations
    carried out by the single-argument variant of kvfree_rcu(), thus avoiding
    this can-sleep code path from dipping into the emergency reserves.
    
    Suggested-by: Michal Hocko <mhocko@suse.com>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 242f0f0..6132452 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3364,7 +3364,8 @@ add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp,
 {
 	struct kvfree_rcu_bulk_data *bnode;
 	bool can_alloc_page = preemptible();
-	gfp_t gfp = (can_sleep ? GFP_KERNEL | __GFP_RETRY_MAYFAIL : GFP_ATOMIC) | __GFP_NOWARN;
+	gfp_t gfp = (can_sleep ? GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOMEMALLOC
+			       : GFP_ATOMIC) | __GFP_NOWARN;
 	int idx;
 
 	*krcp = krc_this_cpu_lock(flags);