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SubjectRe: [PATCH 1/3] perf/core: Make sure the ring-buffer is mapped in all page-tables
On Fri, Jul 20, 2018 at 12:32:10PM -0700, Andy Lutomirski wrote:
> I'm just reading your changelog, and you said the PMDs are no longer
> shared between the page tables. So this presumably means that
> vmalloc_fault() no longer actually works correctly on PTI systems. I
> didn't read the code to figure out *why* it doesn't work, but throwing
> random vmalloc_sync_all() calls around is wrong.

Hmm, so the whole point of vmalloc_fault() fault is to sync changes from
swapper_pg_dir to process page-tables when the relevant parts of the
kernel page-table are not shared, no?

That is also the reason we don't see this on 64 bit, because there these
parts *are* shared.

So with that reasoning vmalloc_fault() works as designed, except that
a warning is issued when it's happens in the NMI path. That warning comes

ebc8827f75954 x86: Barf when vmalloc and kmemcheck faults happen in NMI

which went into 2.6.37 and was added because the NMI handler were not
nesting-safe back then. Reason probably was that the handler on 64 bit
has to use an IST stack and a nested NMI would overwrite the stack of
the upper handler. We don't have this problem on 32 bit as a nested NMI
will not do another stack-switch there.

I am not sure about 64 bit, but there is a lot of assembly magic to make
NMIs nesting-safe, so I guess the problem should be gone there too.



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