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

[Michal Hocko]

On Fri 17-05-19 15:18:19, Alexander Potapenko wrote:
> 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.

No problem

> > 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/)

Not only. GFP_REPEAT being another one and probably others I cannot
remember from the top of my head.

> > 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.

> > 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?

> 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.

Agreed. Over optimization based on an artificial workloads tend to be
dubious IMHO.

-- 
Michal Hocko
SUSE Labs