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

[Andrew Morton <akpm@linux-foundation.org>]

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);
> >
> > I didn't look too closely, but this patch might also address the race
> > which Krzysztof addressed with
> > http://ozlabs.org/~akpm/mmots/broken-out/swap-fix-setting-page_size-blocksize-during-swapoff-swapon-race.patch.
> > Can we please check that out?
> >
> > I do prefer fixing all these swapon-vs-swapoff races with some large,
> > simple, wide-scope exclusion scheme.  Perhaps SWP_USED is that scheme.
> >
> > An alternative would be to add another mutex and just make sys_swapon()
> > and sys_swapoff() 100% exclusive.  But that is plastering yet another
> > lock over this mess to hide the horrors which lurk within :(
> >
> 
> Hi, Andrew. Thanks for your suggestion.
> 
> I checked Krzysztof's patch, it use the global swapon_mutex to protect
> race condition among
> swapon, swapoff and swap_start(). It is a kind of correct method, but
> a heavy method.

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?

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?

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