Re: [PATCH 5/5] perf record: Handle out of space failures writing data with mmap

From: David Ahern <dsahern@gmail.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: acme@ghostprotocols.net, linux-kernel@vger.kernel.org,
	jolsa@redhat.com, Frederic Weisbecker <fweisbec@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Namhyung Kim <namhyung@kernel.org>,
	Mike Galbraith <efault@gmx.de>,
	Stephane Eranian <eranian@google.com>
Subject: Re: [PATCH 5/5] perf record: Handle out of space failures writing data with mmap
Date: Wed, 13 Nov 2013 07:33:52 -0700	[thread overview]
Message-ID: <52838DD0.20906@gmail.com> (raw)
In-Reply-To: <20131112211926.GD25913@gmail.com>

On 11/12/13, 2:19 PM, Ingo Molnar wrote:

> So this isn't very robust, because it assumes that all sources of SIGBUS
> are due to that memcpy() hitting -ENOSPC...
>
> There are several failure modes:
>
>   - If mmap_jmp is not set yet and we get a SIGBUS is some other place,
>     then the longjmp() result will be undefined.
>
>   - If mmap_jmp environment is set, but we've returned from
>     do_mmap_output() already, then the result will be undefined - likely a
>     non-obvious crash.
>
> So at minimum we need a flag that tells us whether the jump environment is
> valid or not - i.e. whether we are executing inside the protected region
> or not - and only do the longjmp() if that flag is set.

Right I meant to add that -- a flag to know when the jmp should be used. 
Got distracted.

>
> Is there really no other way to handle the -ENOSPC case robustly? I guess
> not because the memcpy() really needs memory to write to, but I thought
> I'd ask ...

You need some means to interrupt memcpy and bounce out of it. longjmp is 
the only option I know of.

David