Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.

[Uladzislau Rezki]

On Fri, Oct 02, 2020 at 09:06:24AM +0100, Mel Gorman wrote:
> On Thu, Oct 01, 2020 at 09:26:26PM +0200, Uladzislau Rezki wrote:
> > > 
> > > No, I meant going back to idea of new gfp flag, but adjust the implementation in
> > > the allocator (different from what you posted in previous version) so that it
> > > only looks at the flag after it tries to allocate from pcplist and finds out
> > > it's empty. So, no inventing of new page allocator entry points or checks such
> > > as the one you wrote above, but adding the new gfp flag in a way that it doesn't
> > > affect existing fast paths.
> > >
> > OK. Now i see. Please have a look below at the patch, so we fully understand
> > each other. If that is something that is close to your view or not:
> > 
> > <snip>
> > t a/include/linux/gfp.h b/include/linux/gfp.h
> > index c603237e006c..7e613560a502 100644
> > --- a/include/linux/gfp.h
> > +++ b/include/linux/gfp.h
> > @@ -39,8 +39,9 @@ struct vm_area_struct;
> >  #define ___GFP_HARDWALL                0x100000u
> >  #define ___GFP_THISNODE                0x200000u
> >  #define ___GFP_ACCOUNT         0x400000u
> > +#define ___GFP_NO_LOCKS                0x800000u
> >  #ifdef CONFIG_LOCKDEP
> > -#define ___GFP_NOLOCKDEP       0x800000u
> > +#define ___GFP_NOLOCKDEP       0x1000000u
> >  #else
> >  #define ___GFP_NOLOCKDEP       0
> >  #endif
> > @@ -215,16 +216,22 @@ struct vm_area_struct;
> >   * %__GFP_COMP address compound page metadata.
> >   *
> >   * %__GFP_ZERO returns a zeroed page on success.
> > + *
> > + * %__GFP_NO_LOCKS order-0 allocation without sleepable-locks.
> > + * It obtains a page from the per-cpu-list and considered as
> > + * lock-less. No other actions are performed, thus it returns
> > + * NULL if per-cpu-list is empty.
> >   */
> >  #define __GFP_NOWARN   ((__force gfp_t)___GFP_NOWARN)
> >  #define __GFP_COMP     ((__force gfp_t)___GFP_COMP)
> >  #define __GFP_ZERO     ((__force gfp_t)___GFP_ZERO)
> > +#define __GFP_NO_LOCKS ((__force gfp_t)___GFP_NO_LOCKS)
> > 
> 
> I'm not a fan of the GFP flag approach simply because we've had cases
> before where GFP flags were used in inappropriate contexts like
> __GFP_MEMALLOC which led to a surprising amount of bugs, particularly
> from out-of-tree drivers but also in-tree drivers. Of course, there
> are limited GFP flags available too but at least the comment should
> be as robust as possible. Maybe something like
> 
>  * %__GFP_NO_LOCKS attempts order-0 allocation without sleepable-locks. It
>  * attempts to obtain a page without acquiring any spinlocks. This
>  * should only be used in a context where the holder holds a
>  * raw_spin_lock that cannot be released for the allocation request.
>  * This may be necessary in PREEMPT_RT kernels where a
>  * raw_spin_lock is held which does not sleep tries to acquire a
>  * spin_lock that can sleep with PREEMPT_RT. This should not be
>  * confused with GFP_ATOMIC contexts. Like atomic allocation
>  * requests, there is no guarantee a page will be returned and
>  * the caller must be able to deal with allocation failures.
>  * The risk of allocation failure is higher than using GFP_ATOMIC.
> 
> It's verbose but it would be hard to misinterpret. I think we're
> going to go through a period of time before people get familiar
> with PREEMPT_RT-related hazards as various comments that were
> true are going to be misleading for a while.
> 
Yep, it should be properly documented for sure. Including new GFP_NOWAIT
limitations, same as GFP_ATOMIC once you mentioned.

Thanks!

--
Vlad Rezki