Re: [PATCH] mm/swap: fix race on swap_info reuse between swapoff and swapon

[Mateusz Guzik <mguzik@redhat.com>]

On Mon, Jan 13, 2014 at 11:51:42AM +0800, Weijie Yang wrote:
> On Mon, Jan 13, 2014 at 11:27 AM, Andrew Morton
> <akpm@linux-foundation.org> wrote:
> > On Mon, 13 Jan 2014 11:08:58 +0800 Weijie Yang <weijie.yang.kh@gmail.com> wrote:
> >
> >> >> --- a/mm/swapfile.c
> >> >> +++ b/mm/swapfile.c
> >> >> @@ -1922,7 +1922,6 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
> >> >>       p->swap_map = NULL;
> >> >>       cluster_info = p->cluster_info;
> >> >>       p->cluster_info = NULL;
> >> >> -     p->flags = 0;
> >> >>       frontswap_map = frontswap_map_get(p);
> >> >>       spin_unlock(&p->lock);
> >> >>       spin_unlock(&swap_lock);
> >> >> @@ -1948,6 +1947,16 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
> >> >>               mutex_unlock(&inode->i_mutex);
> >> >>       }
> >> >>       filp_close(swap_file, NULL);
> >> >> +
> >> >> +     /*
> >> >> +     * clear SWP_USED flag after all resources freed
> >> >> +     * so that swapon can reuse this swap_info in alloc_swap_info() safely
> >> >> +     * it is ok to not hold p->lock after we cleared its SWP_WRITEOK
> >> >> +     */
> >> >> +     spin_lock(&swap_lock);
> >> >> +     p->flags = 0;
> >> >> +     spin_unlock(&swap_lock);
> >> >> +
> >> >>       err = 0;
> >> >>       atomic_inc(&proc_poll_event);
> >> >>       wake_up_interruptible(&proc_poll_wait);
> > But do you agree that your
> > http://ozlabs.org/~akpm/mmots/broken-out/mm-swap-fix-race-on-swap_info-reuse-between-swapoff-and-swapon.patch
> > makes Krzysztof's
> > http://ozlabs.org/~akpm/mmots/broken-out/swap-fix-setting-page_size-blocksize-during-swapoff-swapon-race.patch
> > obsolete?
> 
> Yes, I agree.
> 
> > I've been sitting on Krzysztof's
> > swap-fix-setting-page_size-blocksize-during-swapoff-swapon-race.patch
> > for several months - Hugh had issues with it so I put it on hold and
> > nothing further happened.
> >
> >> I will try to resend a patchset to make lock usage in swapfile.c clear
> >> and fine grit
> >
> > OK, thanks.  In the meanwhile I'm planning on dropping Krzysztof's
> > patch and merging your patch into 3.14-rc1, which is why I'd like
> > confirmation that your patch addresses the issues which Krzysztof
> > identified?
> >
> 
> I think so, Krzysztof and I both try to fix the same issue(reuse
> swap_info while its
> previous resources are not cleared completely). The different is
> Krzysztof's patch
> uses a global swapon_mutex and its commit log only focuses on set_blocksize(),
> while my patch try to maintain the fine grit lock usage.
> 

Maybe I should get some sleep first, but I found some minor nits.

Newly introduced window:

p->swap_map == NULL && (p->flags & SWP_USED)

breaks swap_info_get:
        if (!(p->flags & SWP_USED))
                goto bad_device;
        offset = swp_offset(entry);
        if (offset >= p->max)
                goto bad_offset;
        if (!p->swap_map[offset])
                goto bad_free;

so that would need a trivial adjustment.

Another nit is that swap_start and swap_next do the following:
if (!(si->flags & SWP_USED) || !si->swap_map)
	continue;

Testing for swap_map does not look very nice and regardless of your
patch the latter cannot be true if the former is not, thus the check
can be simplified to mere !si->swap_map.

I'm wondering if it would make sense to dedicate a flag (SWP_ALLOCATED?)
to control whether swapon can use give swap_info. That is, it would be
tested and set in alloc_swap_info & cleared like you clear SWP_USED now.
SWP_USED would be cleared as it is and would be set in _enable_swap_info

Then swap_info_get would be left unchanged and swap_* would test for
SWP_USED only.

-- 
Mateusz Guzik

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