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

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Date: Fri, 31 May 2024 14:13:15 +0200
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: Jann Horn <jannh@...gle.com>
Cc: linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
	tglx@...utronix.de, linux-crypto@...r.kernel.org,
	linux-api@...r.kernel.org, x86@...nel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Adhemerval Zanella Netto <adhemerval.zanella@...aro.org>,
	Carlos O'Donell <carlos@...hat.com>,
	Florian Weimer <fweimer@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	Christian Brauner <brauner@...nel.org>,
	David Hildenbrand <dhildenb@...hat.com>, linux-mm@...ck.org
Subject: Re: [PATCH v16 1/5] mm: add VM_DROPPABLE for designating always
 lazily freeable mappings

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@...c4.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.

Jason