Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

[Michal Hocko]

On Mon 10-08-20 18:07:39, Uladzislau Rezki wrote:
> > On Sun 09-08-20 22:43:53, Uladzislau Rezki (Sony) wrote:
> > [...]
> > > Limitations and concerns (Main part)
> > > ====================================
> > > The current memmory-allocation interface presents to following
> > > difficulties that this patch is designed to overcome:
> > > 
> > > a) If built with CONFIG_PROVE_RAW_LOCK_NESTING, the lockdep will
> > >    complain about violation("BUG: Invalid wait context") of the
> > >    nesting rules. It does the raw_spinlock vs. spinlock nesting
> > >    checks, i.e. it is not legal to acquire a spinlock_t while
> > >    holding a raw_spinlock_t.
> > > 
> > >    Internally the kfree_rcu() uses raw_spinlock_t(in rcu-dev branch)
> > >    whereas the "page allocator" internally deals with spinlock_t to
> > >    access to its zones. The code also can be broken from higher level
> > >    of view:
> > >    <snip>
> > >        raw_spin_lock(&some_lock);
> > >        kfree_rcu(some_pointer, some_field_offset);
> > >    <snip>
> > 
> > Is there any fundamental problem to make zone raw_spin_lock?
> > 
> Good point. Converting a regular spinlock to the raw_* variant can solve 
> an issue and to me it seems partly reasonable. Because there are other
> questions if we do it:
> 
> a) what to do with kswapd and "wake-up path" that uses sleepable lock:
>     wakeup_kswapd() -> wake_up_interruptible(&pgdat->kswapd_wait).

If there is no RT friendly variant for waking up process from the atomic
context then we might need to special case this for the RT tree.

> b) How RT people reacts on it? I guess they will no be happy.

zone->lock should be held for a very limited amount of time.

> As i described before, calling the __get_free_page(0) with 0 as argument
> will solve the (a). How correctly is it? From my point of view the logic
> that bypass the wakeup path should be explicitly defined.

gfp_mask == 0 is GFP_NOWAIT (aka an atomic allocation request) which
doesn't wake up kswapd. So if the wakeup is a problem then this would be
a way to go.

> Or we can enter the allocator with (__GFP_HIGH|__GFP_ATOMIC) that bypass
> the __GFP_KSWAPD_RECLAIM as well.

This would be an alternative which consumes memory reserves. Is this
really needed for the particular case?

> 
> Any thoughts here? Please comment.
> 
> Having proposed flag will not heart RT latency and solve all concerns.
> 
> > > b) If built with CONFIG_PREEMPT_RT. Please note, in that case spinlock_t
> > >    is converted into sleepable variant. Invoking the page allocator from
> > >    atomic contexts leads to "BUG: scheduling while atomic".
> > 
> > [...]
> > 
> > > Proposal
> > > ========
> > > 1) Make GFP_* that ensures that the allocator returns NULL rather
> > > than acquire its own spinlock_t. Having such flag will address a and b
> > > limitations described above. It will also make the kfree_rcu() code
> > > common for RT and regular kernel, more clean, less handling corner
> > > cases and reduce the code size.
> > 
> > I do not think this is a good idea. Single purpose gfp flags that tend
> > to heavily depend on the current implementation of the page allocator
> > have turned out to be problematic. Users used to misunderstand their
> > meaning resulting in a lot of abuse which was not trivial to remove.
> > This flag seem to fall into exactly this sort of category. If there is a
> > problem in nesting then that should be addressed rather than a new flag
> > exported IMHO. If that is absolutely not possible for some reason then
> > we can try to figure out what to do but that really need a very strong
> > justification.
> > 
> The problem that i see is we can not use the page allocator from atomic
> contexts, what is our case:
> 
> <snip>
>     local_irq_save(flags) or preempt_disable() or raw_spinlock();
>     __get_free_page(GFP_ATOMIC);
> <snip>
> 
> So if we can convert the page allocator to raw_* lock it will be appreciated,
> at least from our side, IMHO, not from RT one. But as i stated above we need
> to sort raised questions out if converting is done.
> 
> What is your view?

To me it would make more sense to support atomic allocations also for
the RT tree. Having both GFP_NOWAIT and GFP_ATOMIC which do not really
work for atomic context in RT sounds subtle and wrong.
-- 
Michal Hocko
SUSE Labs