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

[Uladzislau Rezki]

On Fri, Oct 02, 2020 at 11:05:07AM +0200, Michal Hocko wrote:
> On Fri 02-10-20 09:50:14, Mel Gorman wrote:
> > On Fri, Oct 02, 2020 at 09:11:23AM +0200, Michal Hocko wrote:
> > > On Thu 01-10-20 21:26:26, 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
> > > 
> > > Even if a new gfp flag gains a sufficient traction and support I am
> > > _strongly_ opposed against consuming another flag for that. Bit space is
> > > limited. 
> > 
> > That is definitely true. I'm not happy with the GFP flag at all, the
> > comment is at best a damage limiting move. It still would be better for
> > a memory pool to be reserved and sized for critical allocations.
> 
> Completely agreed. The only existing usecase is so special cased that a
> dedicated pool is not only easier to maintain but it should be also much
> better tuned for the specific workload. Something not really feasible
> with the allocator.
> 
> > > Besides that we certainly do not want to allow craziness like
> > > __GFP_NO_LOCK | __GFP_RECLAIM (and similar), do we?
> > 
> > That would deserve to be taken to a dumpster and set on fire. The flag
> > combination could be checked in the allocator but the allocator path fast
> > paths are bad enough already.
> 
> If a new allocation/gfp mode is absolutely necessary then I believe that
> the most reasoanble way forward would be
> #define GFP_NO_LOCK	((__force gfp_t)0)
> 
Agree. Even though i see that some code should be adjusted for it. There are
a few users of the __get_free_page(0); So, need to double check it:

<snip>
[    0.650351] BUG: kernel NULL pointer dereference, address: 0000000000000010
[    0.651083] #PF: supervisor read access in kernel mode
[    0.651639] #PF: error_code(0x0000) - not-present page
[    0.652200] PGD 0 P4D 0
[    0.652523] Oops: 0000 [#1] SMP NOPTI
[    0.652668] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.9.0-rc7-next-20200930+ #140
[    0.652668] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
[    0.652668] RIP: 0010:__find_event_file+0x21/0x80
<snip>

Apart of that. There is a post_alloc_hook(), that gets called from the prep_new_page().
If "debug page alloc enabled", it maps a page for debug purposes invoking kernel_map_pages().
__kernel_map_pages() is ARCH specific. For example, powerpc variant uses sleep-able locks
what can be easily converted to raw variant. 

--
Vlad Rezki