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