Re: [PATCH v2] mm/swap: fix swap_info_struct race between swapoff and get_swap_pages()

[Rongwei Wang]

On 2023/4/6 22:57, Aaron Lu wrote:
> On Thu, Apr 06, 2023 at 10:04:16PM +0800, Aaron Lu wrote:
> > On Tue, Apr 04, 2023 at 11:47:16PM +0800, Rongwei Wang wrote:
> > > The si->lock must be held when deleting the si from
> > > the available list.  Otherwise, another thread can
> > > re-add the si to the available list, which can lead
> > > to memory corruption. The only place we have found
> > > where this happens is in the swapoff path. This case
> > > can be described as below:
> > >
> > > core 0                       core 1
> > > swapoff
> > >
> > > del_from_avail_list(si)      waiting
> > >
> > > try lock si->lock            acquire swap_avail_lock
> > >                               and re-add si into
> > >                               swap_avail_head
> >                                 confused here.
> >
> > If del_from_avail_list(si) finished in swaoff path, then this si should
> > not exist in any of the per-node avail list and core 1 should not be
> > able to re-add it.
> I think a possible sequence could be like this:
>
> cpuX                             cpuY
> swapoff                          put_swap_folio()
>
> del_from_avail_list(si)
>                                   taken si->lock
> spin_lock(&si->lock);
>
> 				 swap_range_free()
> 				 was_full && SWP_WRITEOK -> re-add!
> 				 drop si->lock
>
> taken si->lock
> proceed removing si
>
> End result: si left on avail_list after being swapped off.
>
> The problem is, in add_to_avail_list(), it has no idea this si is being
> swapped off and taking si->lock then del_from_avail_list() could avoid
> this problem, so I think this patch did the right thing but the
> changelog about how this happened needs updating and after that:

Hi Aaron

That's my fault. Actually, I don't refers specifically to 
swap_range_free() path in commit, mainly because cpuY can stand all 
threads which is waiting swap_avail_lock, then to call 
add_to_avail_list(), not only swap_range_free(), e.g. maybe swapon().

> Reviewed-by: Aaron Lu <aaron.lu@xxxxxxxxx>

Thanks for your time.

-wrw

> Thanks,
> Aaron