Re: [PATCH v2 3/4] gfp: mm: introduce __GFP_NO_AUTOINIT

[Michal Hocko <mhocko@kernel.org>]

On Tue 21-05-19 16:18:37, Alexander Potapenko wrote:
> On Fri, May 17, 2019 at 7:11 PM Michal Hocko <mhocko@kernel.org> wrote:
> >
> > On Fri 17-05-19 09:27:54, Kees Cook wrote:
> > > On Fri, May 17, 2019 at 04:01:08PM +0200, Michal Hocko wrote:
> > > > On Fri 17-05-19 15:37:14, Alexander Potapenko wrote:
> > > > > > > > Freeing a memory is an opt-in feature and the slab allocator can already
> > > > > > > > tell many (with constructor or GFP_ZERO) do not need it.
> > > > > > > Sorry, I didn't understand this piece. Could you please elaborate?
> > > > > >
> > > > > > The allocator can assume that caches with a constructor will initialize
> > > > > > the object so additional zeroying is not needed. GFP_ZERO should be self
> > > > > > explanatory.
> > > > > Ah, I see. We already do that, see the want_init_on_alloc()
> > > > > implementation here: https://patchwork.kernel.org/patch/10943087/
> > > > > > > > So can we go without this gfp thing and see whether somebody actually
> > > > > > > > finds a performance problem with the feature enabled and think about
> > > > > > > > what can we do about it rather than add this maint. nightmare from the
> > > > > > > > very beginning?
> > > > > > >
> > > > > > > There were two reasons to introduce this flag initially.
> > > > > > > The first was double initialization of pages allocated for SLUB.
> > > > > >
> > > > > > Could you elaborate please?
> > > > > When the kernel allocates an object from SLUB, and SLUB happens to be
> > > > > short on free pages, it requests some from the page allocator.
> > > > > Those pages are initialized by the page allocator
> > > >
> > > > ... when the feature is enabled ...
> > > >
> > > > > and split into objects. Finally SLUB initializes one of the available
> > > > > objects and returns it back to the kernel.
> > > > > Therefore the object is initialized twice for the first time (when it
> > > > > comes directly from the page allocator).
> > > > > This cost is however amortized by SLUB reusing the object after it's been freed.
> > > >
> > > > OK, I see what you mean now. Is there any way to special case the page
> > > > allocation for this feature? E.g. your implementation tries to make this
> > > > zeroying special but why cannot you simply do this
> > > >
> > > >
> > > > struct page *
> > > > ____alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid,
> > > >                                                     nodemask_t *nodemask)
> > > > {
> > > >     //current implementation
> > > > }
> > > >
> > > > struct page *
> > > > __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid,
> > > >                                                     nodemask_t *nodemask)
> > > > {
> > > >     if (your_feature_enabled)
> > > >             gfp_mask |= __GFP_ZERO;
> > > >     return ____alloc_pages_nodemask(gfp_mask, order, preferred_nid,
> > > >                                     nodemask);
> > > > }
> > > >
> > > > and use ____alloc_pages_nodemask from the slab or other internal
> > > > allocators?
> Given that calling alloc_pages() with __GFP_NO_AUTOINIT doesn't
> visibly improve the chosen benchmarks,
> and the next patch in the series ("net: apply __GFP_NO_AUTOINIT to
> AF_UNIX sk_buff allocations") only improves hackbench,
> shall we maybe drop both patches altogether?

Ohh, by all means. I was suggesting the same few emails ago. The above
is just a hint on how to implement the feature on the page allocator
level rather than hooking into the prep_new_page and add another branch
to zero memory.
-- 
Michal Hocko
SUSE Labs