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

[Alexander Potapenko]

On Fri, May 17, 2019 at 2:59 PM Michal this flag Hocko
<mhocko at kernel.org> wrote:
>
> [It would be great to keep people involved in the previous version in the
> CC list]
Yes, I've been trying to keep everyone in the loop, but your email
fell through the cracks.
Sorry about that.
> On Tue 14-05-19 16:35:36, Alexander Potapenko wrote:
> > When passed to an allocator (either pagealloc or SL[AOU]B),
> > __GFP_NO_AUTOINIT tells it to not initialize the requested memory if the
> > init_on_alloc boot option is enabled. This can be useful in the cases
> > newly allocated memory is going to be initialized by the caller right
> > away.
> >
> > __GFP_NO_AUTOINIT doesn't affect init_on_free behavior, except for SLOB,
> > where init_on_free implies init_on_alloc.
> >
> > __GFP_NO_AUTOINIT basically defeats the hardening against information
> > leaks provided by init_on_alloc, so one should use it with caution.
> >
> > This patch also adds __GFP_NO_AUTOINIT to alloc_pages() calls in SL[AOU]B.
> > Doing so is safe, because the heap allocators initialize the pages they
> > receive before passing memory to the callers.
>
> I still do not like the idea of a new gfp flag as explained in the
> previous email. People will simply use it incorectly or arbitrarily.
> We have that juicy experience from the past.

Just to preserve some context, here's the previous email:
https://patchwork.kernel.org/patch/10907595/
(plus the patch removing GFP_TEMPORARY for the curious ones:
https://lwn.net/Articles/729145/)

> 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?

> 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.
However the benchmark results provided in this and the previous patch
don't show any noticeable difference - most certainly because the cost
of initializing the page is amortized.
The second one was to fine-tune hackbench, for which the slowdown
drops by a factor of 2.
But optimizing a mitigation for certain benchmarks is a questionable
measure, so maybe we could really go without it.

Kees, what do you think?
> --
> Michal Hocko
> SUSE Labs



-- 
Alexander Potapenko
Software Engineer

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