Re: [PATCH v16 1/5] mm: add VM_DROPPABLE for designating always lazily freeable mappings

Jann Horn
2024-06-07T15:13:16Z

On Fri, Jun 7, 2024 at 4:35 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> On Fri, May 31, 2024 at 03:00:26PM +0200, Jann Horn wrote:
> > On Fri, May 31, 2024 at 2:13 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> > > On Fri, May 31, 2024 at 12:48:58PM +0200, Jann Horn wrote:
> > > > On Tue, May 28, 2024 at 2:24 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> > > > > c) If there's not enough memory to service a page fault, it's not fatal.
> > > > [...]
> > > > > @@ -5689,6 +5689,10 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
> > > > >
> > > > > lru_gen_exit_fault();
> > > > >
> > > > > + /* If the mapping is droppable, then errors due to OOM aren't fatal. */
> > > > > + if (vma->vm_flags & VM_DROPPABLE)
> > > > > + ret &= ~VM_FAULT_OOM;
> > > >
> > > > Can you remind me how this is supposed to work? If we get an OOM
> > > > error, and the error is not fatal, does that mean we'll just keep
> > > > hitting the same fault handler over and over again (until we happen to
> > > > have memory available again I guess)?
> > >
> > > Right, it'll just keep retrying. I agree this isn't great, which is why
> > > in the 2023 patchset, I had additional code to simply skip the faulting
> > > instruction, and then the userspace code would notice the inconsistency
> > > and fallback to the syscall. This worked pretty well. But it meant
> > > decoding the instruction and in general skipping instructions is weird,
> > > and that made this patchset very very contentious. Since the skipping
> > > behavior isn't actually required by the /security goals/ of this, I
> > > figured I'd just drop that. And maybe we can all revisit it together
> > > sometime down the line. But for now I'm hoping for something a little
> > > easier to swallow.
> >
> > In that case, since we need to be able to populate this memory to make
> > forward progress, would it make sense to remove the parts of the patch
> > that treat the allocation as if it was allowed to silently fail (the
> > "__GFP_NOWARN | __GFP_NORETRY" and the "ret &= ~VM_FAULT_OOM")? I
> > think that would also simplify this a bit by making this type of
> > memory a little less special.
>
> The whole point, though, is that it needs to not fail or warn. It's
> memory that can be dropped/zeroed at any moment, and the code is
> deliberately robust to that.

Sure - but does it have to be more robust than accessing a newly
allocated piece of memory [which hasn't been populated with anonymous
pages yet] or bringing a swapped-out page back from swap?

I'm not an expert on OOM handling, but my understanding is that the
kernel tries _really_ hard to avoid failing low-order GFP_KERNEL
allocations, with the help of the OOM killer. My understanding is that
those allocations basically can't fail with a NULL return unless the
process has already been killed or it is in a memcg_kmem cgroup that
contains only processes that have been marked as exempt from OOM
killing. (Or if you're using error injection to explicitly tell the
kernel to fail the allocation.)
My understanding is that normal outcomes of an out-of-memory situation
are things like the OOM killer killing processes (including
potentially the calling one) to free up memory, or the OOM killer
panic()ing the whole system as a last resort; but getting a NULL
return from page_alloc(GFP_KERNEL) without getting killed is not one
of those outcomes.