Re: [RFC][PATCH] x86/mm: Sync all vmalloc mappings before text_poke()

Mathieu Desnoyers
2020-04-30T15:20:18Z

----- On Apr 30, 2020, at 10:50 AM, Joerg Roedel jroedel@suse.de wrote:

> On Thu, Apr 30, 2020 at 04:11:20PM +0200, Joerg Roedel wrote:
>> The page-fault handler calls a tracing function which again ends up in
>> trace_event_ignore_this_pid(), where it faults again. From here on the CPU is in
>> a page-fault loop, which continues until the stack overflows (with
>> CONFIG_VMAP_STACK).
>
> Did some more testing to find out what this issue has to do with
>
> 763802b53a42 x86/mm: split vmalloc_sync_all()
>
> Above commit removes a call to vmalloc_sync_all() from the vmalloc
> unmapping path, because that call caused severe performance regressions
> on some workloads and was not needed on x86-64 anyway.
>
> But that call caused vmalloc_sync_all() to be called regularily on
> x86-64 machines, so that all page-tables were more likely to be in sync.
>
> The call was introduced by commit
>
> 3f8fd02b1bf1 mm/vmalloc: Sync unmappings in __purge_vmap_area_lazy()
>
> to fix a correctness issue on x86-32 PAE systems, which also need
> unmappings of large pages in the vmalloc area to be synchronized.
>
> This additional call to vmalloc_sync_all() did hide the problem. I
> verified it by reverting both of the above commits on v5.7-rc3 and
> testing on that kernel. The problem is reproducible there too, the box
> hangs hard.
>
> So the underlying problem is that a vmalloc()'ed tracing buffer is used
> to trace the page-fault handler, so that it has no chance of faulting in
> the buffer address to poking_mm and maybe other PGDs.
>
> The right fix is to call vmalloc_sync_mappings() right after allocating
> tracing or perf buffers via v[zm]alloc().

Either right after allocation, or right before making the vmalloc'd data
structure visible to the instrumentation. In the case of the pid filter,
that would be the rcu_assign_pointer() which publishes the new pid filter
table.

As long as vmalloc_sync_mappings() is performed somewhere *between* allocation
and publishing the pointer for instrumentation, it's fine.

I'll let Steven decide on which approach works best for him.

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.