linux-kernel - Re: increased vmap_area_lock contentions on "n_tty: Move buffers into n_tty

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Message-ID: <52441FF7.1030405@hurleysoftware.com>
Date:	Thu, 26 Sep 2013 07:52:23 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Andi Kleen <andi@...stfloor.org>, Lin Ming <minggr@...il.com>
CC:	Fengguang Wu <fengguang.wu@...el.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	LKML <linux-kernel@...r.kernel.org>, lkp@...org,
	Tejun Heo <tj@...nel.org>
Subject: Re: increased vmap_area_lock contentions on "n_tty: Move buffers
 into n_tty_data"

On 09/25/2013 11:20 PM, Andi Kleen wrote:
> Lin Ming <minggr@...il.com> 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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