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

[Michal Hocko <mhocko@suse.com>]

On Tue 29-09-20 18:53:27, Paul E. McKenney wrote:
> On Tue, Sep 29, 2020 at 02:07:56PM +0200, Michal Hocko wrote:
> > On Mon 28-09-20 16:31:01, paulmck@kernel.org wrote:
> > [...]
> 
> Apologies for the delay, but today has not been boring.
> 
> > > This commit therefore uses preemptible() to determine whether allocation
> > > is possible at all for double-argument kvfree_rcu().
> > 
> > This deserves a comment. Because GFP_ATOMIC is possible for many
> > !preemptible() contexts. It is the raw_spin_lock, NMIs and likely few
> > others that are a problem. You are taking a conservative approach which
> > is fine but it would be good to articulate that explicitly.
> 
> Good point, and so I have added the following as a header comment to
> the add_ptr_to_bulk_krc_lock() function:
> 
> // Record ptr in a page managed by krcp, with the pre-krc_this_cpu_lock()
> // state specified by flags.  If can_sleep is true, the caller must
> // be schedulable and not be holding any locks or mutexes that might be
> // acquired by the memory allocator or anything that it might invoke.
> // If !can_sleep, then if !preemptible() no allocation will be undertaken,
> // otherwise the allocation will use GFP_ATOMIC to avoid the remainder of
> // the aforementioned deadlock possibilities.  Returns true iff ptr was
> // successfully recorded, else the caller must use a fallback.

OK, not trivial to follow but at least verbose enough to understand the
intention after some mulling. Definitely an improvement, thanks!

[...]
> > > -kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
> > > +add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp,
> > > +	unsigned long *flags, void *ptr, bool can_sleep)
> > >  {
> > >  	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;
> > 
> > This is quite confusing IMHO. At least without a further explanation.
> > can_sleep is not as much about sleeping as it is about the reclaim
> > recursion AFAIU your changelog, right?
> 
> 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.

[...]

> > 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.
-- 
Michal Hocko
SUSE Labs