Re: [PATCH v14 2/7] mm: add VM_DROPPABLE for designating always lazily freeable mappings

Andy Lutomirski
2023-01-03T20:52:31Z

On Tue, Jan 3, 2023 at 11:06 AM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> Hi Andy,
>
> Thanks for your constructive suggestions.
>
> On Tue, Jan 03, 2023 at 10:36:01AM -0800, Andy Lutomirski wrote:
> > > > c) If there's not enough memory to service a page fault, it's not fatal,
> > > > and no signal is sent. Instead, writes are simply lost.
> >
> > This just seems massively overcomplicated to me. If there isn't
> > enough memory to fault in a page of code, we don't have some magic
> > instruction emulator in the kernel. We either OOM or we wait for
> > memory to show up.
>
> Before addressing the other parts of your email, I thought I'd touch on
> this. Quoting from the email I just wrote Ingo:
>
> | *However* - if your big objection to this patch is that the instruction
> | skipping is problematic, we could actually punt that part. The result
> | will be that userspace just retries the memory write and the fault
> | happens again, and eventually it succeeds. From a perspective of
> | vgetrandom(), that's perhaps worse -- with this v14 patchset, it'll
> | immediately fallback to the syscall under memory pressure -- but you
> | could argue that nobody really cares about performance at that point
> | anyway, and so just retrying the fault until it succeeds is a less
> | complex behavior that would be just fine.
> |
> | Let me know if you think that'd be an acceptable compromise, and I'll
> | roll it into v15. As a preview, it pretty much amounts to dropping 3/7
> | and editing the commit message in this 2/7 patch.
>
> IOW, I think the main ideas of the patch work just fine without "point
> c" with the instruction skipping. Instead, waiting/retrying could
> potentially work. So, okay, it seems like the two of you both hate the
> instruction decoder stuff, so I'll plan on working that part in, in one
> way or another, for v15.
>
> > On Tue, Jan 3, 2023 at 2:50 AM Ingo Molnar <mingo@kernel.org> wrote:
> > > > The vDSO getrandom() implementation works with a buffer allocated with a
> > > > new system call that has certain requirements:
> > > >
> > > > - It shouldn't be written to core dumps.
> > > > * Easy: VM_DONTDUMP.
> > > > - It should be zeroed on fork.
> > > > * Easy: VM_WIPEONFORK.
> >
> > I have a rather different suggestion: make a special mapping. Jason,
> > you're trying to shoehorn all kinds of bizarre behavior into the core
> > mm, and none of that seems to me to belong to the core mm. Instead,
> > have an actual special mapping with callbacks that does the right
> > thing. No fancy VM flags.
>
> Oooo! I like this. Avoiding adding VM_* flags would indeed be nice.
> I had seen things that I thought looked in this direction with the shmem
> API, but when I got into the details, it looked like this was meant for
> something else and couldn't address most of what I wanted here.
>
> If you say this is possible, I'll look again to see if I can figure it
> out. Though, if you have some API name at the top of your head, you
> might save me some code squinting time.

Look for _install_special_mapping().

--Andy

> > Want to mlock it? No, don't do that -- that's absurd. Just arrange
> > so that, if it gets evicted, it's not written out anywhere. And when
> > it gets faulted back in it does the right thing -- see above.
>
> Presumably mlock calls are redirected to some function pointer so I can
> just return EINTR?

Or just don't worry about it. If someone mlocks() it, that's their
problem. The point is that no one needs to.

>
> > Zero on fork? I'm sure that's manageable with a special mapping. If
> > not, you can add a new vm operation or similar to make it work. (Kind
> > of like how we extended special mappings to get mremap right a couple
> > years go.) But maybe you don't want to *zero* it on fork and you want
> > to do something more intelligent. Fine -- you control ->fault!
>
> Doing something more intelligent would be an interesting development, I
> guess... But, before I think about that, all mapping have flags;
> couldn't I *still* set VM_WIPEONFORK on the special mapping? Or does the
> API you have in mind not work that way? (Side note: I also want
> VM_DONTDUMP to work.)

You really want unmap (the pages, not the vma) on fork, not wipe on
fork. It'll be VM_SHARED, and I'm not sure what VM_WIPEONFORK |
VM_SHARED does.