Re: [RFC PATCH] mm: introduce kv[mz]alloc helpers

Michal Hocko
2016-12-20T13:50:16Z

On Wed 14-12-16 09:59:16, Michal Hocko wrote:
> On Tue 13-12-16 14:07:33, Joe Perches wrote:
> > On Tue, 2016-12-13 at 11:14 +0100, Michal Hocko wrote:
> > > Are there any more comments or objections to this patch? Is this a good
> > > start or kv[mz]alloc has to provide a way to cover GFP_NOFS users as
> > > well in the initial version.
> >
> > Did Andrew Morton ever comment on this?
> > I believe he was the primary objector in the past.
> >
> > Last I recollect was over a year ago:
> >
> > https://lkml.org/lkml/2015/7/7/1050
>
> Let me quote:
> : Sigh. We've resisted doing this because vmalloc() is somewhat of a bad
> : thing, and we don't want to make it easy for people to do bad things.
> :
> : And vmalloc is bad because a) it's slow and b) it does GFP_KERNEL
> : allocations for page tables and c) it is susceptible to arena
> : fragmentation.
> :
> : We'd prefer that people fix their junk so it doesn't depend upon large
> : contiguous allocations. This isn't userspace - kernel space is hostile
> : and kernel code should be robust.
> :
> : So I dunno. Should we continue to make it a bit more awkward to use
> : vmalloc()? Probably that tactic isn't being very successful - people
> : will just go ahead and open-code it. And given the surprising amount
> : of stuff you've placed in kvmalloc_node(), they'll implement it
> : incorrectly...
> :
> : How about we compromise: add kvmalloc_node(), but include a BUG_ON("you
> : suck") to it?
>
> While I agree with some of those points, the reality really sucks,
> though. We have tried the same tactic with __GFP_NOFAIL and failed as
> well. I guess we should just bite the bullet and provide an api which is
> so common that people keep reinventing their own ways around that, many
> times wrongly or suboptimally. BUG_ON("you suck") is just not going to
> help much I am afraid.
>
> What do you think Andrew?

So what are we going to do about this patch?