Re: increased vmap_area_lock contentions on "n_tty: Move buffersinto n_tty_data"

[Peter Hurley]

On 09/25/2013 11:20 PM, Andi Kleen wrote:
> Lin Ming <minggr@xxxxxxxxx> writes:
> > Would you like below patch?
>
> The loop body keeps rather complex state. It could easily
> get confused by parallel RCU changes.
>
> So if the list changes in parallel you may suddenly
> report very bogus values, as the va_start - prev_end
> computation may be bogus.
>
> Perhaps it's ok (may report bogus gaps), but it seems a bit risky.

I don't understand how the computed gap would be bogus; there
_was_ a list state in which that particular gap existed. The fact
that it may not exist anymore can also happen in the existing
algorithm the instant get_vmalloc_info() drops the vmap_area_lock.

OTOH, parallel list changes could cause an rcu-based get_vmalloc_info()
to over-report or under-report used memory due to parallel list
changes.

If this is a problem in practice, then usage and largest chunk
should be tracked by the allocator instead, obviating the need for
get_vmalloc_info() to traverse the vmap_area_list at all.

Regards,
Peter Hurley
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