Messages in this thread | | | From | Linus Torvalds <> | Date | Wed, 27 Oct 2021 14:14:48 -0700 | Subject | Re: [PATCH v8 00/17] gfs2: Fix mmap + page fault deadlocks |
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On Wed, Oct 27, 2021 at 12:13 PM Catalin Marinas <catalin.marinas@arm.com> 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
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