Re: [PATCH v19 5/8] mm: introduce memfd_secret system call to create "secret" memory areas

[David Hildenbrand]

On 18.05.21 12:31, Michal Hocko wrote:
> On Tue 18-05-21 12:06:42, David Hildenbrand wrote:
> > On 18.05.21 11:59, Michal Hocko wrote:
> > > On Sun 16-05-21 10:29:24, Mike Rapoport wrote:
> > > > On Fri, May 14, 2021 at 11:25:43AM +0200, David Hildenbrand wrote:
> > > [...]
> > > > > > +		if (!page)
> > > > > > +			return VM_FAULT_OOM;
> > > > > > +
> > > > > > +		err = set_direct_map_invalid_noflush(page, 1);
> > > > > > +		if (err) {
> > > > > > +			put_page(page);
> > > > > > +			return vmf_error(err);
> > > > >
> > > > > Would we want to translate that to a proper VM_FAULT_..., which would most
> > > > > probably be VM_FAULT_OOM when we fail to allocate a pagetable?
> > > >
> > > > That's what vmf_error does, it translates -ESOMETHING to VM_FAULT_XYZ.
> > >
> > > I haven't read through the rest but this has just caught my attention.
> > > Is it really reasonable to trigger the oom killer when you cannot
> > > invalidate the direct mapping. From a quick look at the code it is quite
> > > unlikely to se ENOMEM from that path (it allocates small pages) but this
> > > can become quite sublte over time. Shouldn't this simply SIGBUS if it
> > > cannot manipulate the direct mapping regardless of the underlying reason
> > > for that?
> >
> > OTOH, it means our kernel zones are depleted, so we'd better reclaim somehow
> > ...
>
> Killing a userspace seems to be just a bad way around that.
>
> Although I have to say openly that I am not a great fan of VM_FAULT_OOM
> in general. It is usually a a wrong way to tell the handle the failure
> because it happens outside of the allocation context so you lose all the
> details (e.g. allocation constrains, numa policy etc.). Also whenever
> there is ENOMEM then the allocation itself has already made sure that
> all the reclaim attempts have been already depleted. Just consider an
> allocation with GFP_NOWAIT/NO_RETRY or similar to fail and propagate
> ENOMEM up the call stack. Turning that into the OOM killer sounds like a
> bad idea to me.  But that is a more general topic. I have tried to bring
> this up in the past but there was not much of an interest to fix it as
> it was not a pressing problem...

I'm certainly interested; it would mean that we actually want to try 
recovering from VM_FAULT_OOM in various cases, and as you state, we 
might have to supply more information to make that work reliably.

Having that said, I guess what we have here is just the same as when our 
process fails to allocate a generic page table in __handle_mm_fault(), 
when we fail p4d_alloc() and friends ...

--
Thanks,

David / dhildenb