Re: [PATCH v8 00/17] gfs2: Fix mmap + page fault deadlocks

Linus Torvalds <[email protected]> Wed, 27 Oct 2021 21:14:48 +0000

Newsgroups	org.kernel.vger.kvm-ppc,com.redhat.cluster-devel,dev.linux.lists.ocfs2-devel,org.kernel.vger.linux-btrfs,org.kernel.vger.linux-fsdevel,org.kernel.vger.linux-kernel
Message-ID	<CAHk-=wjQqi9cw1Guz6a8oBB0xiQNF_jtFzs3gW0k7+fKN-mB1g@mail.gmail.com>

On Wed, Oct 27, 2021 at 12:13 PM Catalin Marinas
<[email protected]> wrote:
>
> As an alternative, you mentioned earlier that a per-thread fault status
> was not feasible on x86 due to races. Was this only for the hw poison
> case? I think the uaccess is slightly different.

It's not x86-specific, it's very generic.

If we set some flag in the per-thread status, we'll need to be careful
about not overwriting it if we then have a subsequent NMI that _also_
takes a (completely unrelated) page fault - before we then read the
per-thread flag.

Think 'perf' and fetching backtraces etc.

Note that the NMI page fault can easily also be a pointer coloring
fault on arm64, for exactly the same reason that whatever original
copy_from_user() code was. So this is not a "oh, pointer coloring
faults are different". They have the same re-entrancy issue.

And both the "pagefault_disable" and "fault happens in interrupt
context" cases are also the exact same 'faulthandler_disabled()'
thing. So even at fault time they look very similar.

So we'd have to have some way to separate out only the one we care about.

               Linus