linux-kernel - Re: [PATCH] frontswap: enable call to invalidate area on swapoff

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20131009144045.GA5406@variantweb.net>
Date:	Wed, 9 Oct 2013 09:40:45 -0500
From:	Seth Jennings <spartacus06@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Krzysztof Kozlowski <k.kozlowski@...sung.com>, linux-mm@...ck.org,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	linux-kernel@...r.kernel.org, Shaohua Li <shli@...ionio.com>,
	Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH] frontswap: enable call to invalidate area on swapoff

On Tue, Oct 08, 2013 at 01:08:53PM -0700, Andrew Morton wrote:
> On Tue, 08 Oct 2013 10:13:20 +0200 Krzysztof Kozlowski <k.kozlowski@...sung.com> wrote:
> 
> > On pon, 2013-10-07 at 15:03 -0700, Andrew Morton wrote:
> > > On Mon, 07 Oct 2013 17:25:41 +0200 Krzysztof Kozlowski <k.kozlowski@...sung.com> wrote:
> > > 
> > > > During swapoff the frontswap_map was NULL-ified before calling
> > > > frontswap_invalidate_area(). However the frontswap_invalidate_area()
> > > > exits early if frontswap_map is NULL. Invalidate was never called during
> > > > swapoff.
> > > > 
> > > > This patch moves frontswap_map_set() in swapoff just after calling
> > > > frontswap_invalidate_area() so outside of locks
> > > > (swap_lock and swap_info_struct->lock). This shouldn't be a problem as
> > > > during swapon the frontswap_map_set() is called also outside of any
> > > > locks.
> > > > 
> > > 
> > > Ahem.  So there's a bunch of code in __frontswap_invalidate_area()
> > > which hasn't ever been executed and nobody noticed it.  So perhaps that
> > > code isn't actually needed?
> > > 
> > > More seriously, this patch looks like it enables code which hasn't been
> > > used or tested before.  How well tested was this?
> > > 
> > > Are there any runtime-visible effects from this change?
> > 
> > I tested zswap on x86 and x86-64 and there was no difference. This is
> > good as there shouldn't be visible anything because swapoff is unusing
> > all pages anyway:
> > 	try_to_unuse(type, false, 0); /* force all pages to be unused */
> > 
> > I haven't tested other frontswap users.
> 
> So is that code in __frontswap_invalidate_area() unneeded?

Yes, to expand on what Bob said, __frontswap_invalidate_area() is still
needed to let any frontswap backend free per-swaptype resources.

__frontswap_invalidate_area() is _not_ for freeing structures associated
with individual swapped out pages since all of the pages should be
brought back into memory by try_to_unuse() before
__frontswap_invalidate_area() is called.

The reason we never noticed this for zswap is that zswap has no
dynamically allocated per-type resources.  In the expected case,
where all of the pages have been drained from zswap,
zswap_frontswap_invalidate_area() is a no-op.

Seth
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/