Re: About swapoff race patch (was Re: [PATCH] mm, swap: bounds check swap_info accesses to avoid NU

["Huang\, Ying" <ying.huang@intel.com>]

Andrea Parri <andrea.parri@amarulasolutions.com> writes:

> Hi Huang, Ying,
>
> On Mon, Feb 04, 2019 at 01:37:00PM -0800, Hugh Dickins wrote:
>> On Thu, 31 Jan 2019, Andrew Morton wrote:
>> > On Thu, 31 Jan 2019 10:48:29 +0800 "Huang\, Ying" <ying.huang@intel.com> wrote:
>> > > Andrew Morton <akpm@linux-foundation.org> writes:
>> > > > mm-swap-fix-race-between-swapoff-and-some-swap-operations.patch is very
>> > > > stuck so can you please redo this against mainline?
>> > > 
>> > > Allow me to be off topic, this patch has been in mm tree for quite some
>> > > time, what can I do to help this be merged upstream?
>
> [...]
>
>
>> 
>> Wow, yes, it's about a year old.
>> 
>> > 
>> > I have no evidence that it has been reviewed, for a start.  I've asked
>> > Hugh to look at it.
>> 
>> I tried at the weekend.  Usual story: I don't like it at all, the
>> ever-increasing complexity there, but certainly understand the need
>> for that fix, and have not managed to think up anything better -
>> and now I need to switch away, sorry.
>
> FWIW, I do agree with Hugh about "the need for that fix": AFAIU, that
> (mainline) code is naively buggy _and_ "this patch":
>
>   http://lkml.kernel.org/r/20180223060010.954-1-ying.huang@intel.com
>
> "redone on top of mainline" seems both correct and appropriate to me.

Thanks!  Because the patch needs to go through -mm tree, so I will
rebase the patch on top of the head of -mm tree.
>
>> (I was originally horrified by the stop_machine() added in swapon and
>> swapoff, but perhaps I'm remembering a distant past of really stopping
>> the machine: stop_machine() today looked reasonable, something to avoid
>> generally like lru_add_drain_all(), but not as shameful as I thought.)
>
> AFAIC_find_on_LKML, we have three different fixes (at least!): resp.,
>
>   1. refcount(-based),
>   2. RCU,
>   3. stop_machine();
>
> (3) appears to be the less documented/relied-upon/tested among these;
> I'm not aware of definitive reasons forcing us to reject (1) and (2).

Because swapoff() is a really cold path, while page fault handler is a
really hot path.  (3) can minimize the overhead of the hot path.

Best Regards,
Huang, Ying

>   Andrea
>
>
>> 
>> Hugh