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

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Message-ID: <20150407175838.GA2898@codemonkey.org.uk>
Date:	Tue, 7 Apr 2015 13:58:38 -0400
From:	Dave Jones <davej@...emonkey.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	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>,
	the arch/x86 maintainers <x86@...nel.org>
Subject: Re: Hang on large copy_from_user with PREEMPT_NONE

On Tue, Apr 07, 2015 at 10:33:49AM -0700, Linus Torvalds wrote:
 > 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.
 > 
 > +#define COPY_CHUNK_SIZE (16*PAGE_SIZE)

This needs to be bigger, just to accomodate things like xfs, which
on my system is over 1mb.  I think some out-of-tree modules
(not that I care deeply) are even bigger.

Special casing might be an easier call.

	Dave

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