linux-kernel - Re: Hang on large copy_from_user with PREEMPT

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Date:	Tue, 7 Apr 2015 10:33:49 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Ingo Molnar <mingo@...nel.org>, Michal Hocko <mhocko@...e.cz>,
	Borislav Petkov <bp@...en8.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Dave Jones <davej@...hat.com>,
	"the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: Hang on large copy_from_user with PREEMPT_NONE

On Tue, Apr 7, 2015 at 10:00 AM, Sasha Levin <sasha.levin@...cle.com> wrote:
>
> It locks up quickly without KASan as well.

I suspect it's some virtualization artifact, where the writes cause
COW faults (or just memory allocations) in the L0 domain.

Whatever. It's probably not worth fighting. Either we just decide that
"copy_from_user()" shouldn't bother to zero huge areas (limit the
zeroing to some arbitrary size), or we just special-case the module
loading.

Something like the attached patch.

Entirely untested, as usual.

                      Linus

View attachment "patch.diff" of type "text/plain" (1112 bytes)