Re: [Patch v2] mm: thp: grab the lock before manipulation defer list

[Wei Yang]

On Tue, Jan 14, 2020 at 01:59:21PM +0300, Kirill A. Shutemov wrote:
>On Tue, Jan 14, 2020 at 01:31:12PM +0300, Kirill A. Shutemov wrote:
>> On Tue, Jan 14, 2020 at 10:31:22AM +0100, Michal Hocko wrote:
>> > On Sat 11-01-20 03:03:52, Kirill A. Shutemov wrote:
>> > > On Thu, Jan 09, 2020 at 10:30:54PM +0800, Wei Yang wrote:
>> > > > As all the other places, we grab the lock before manipulate the defer list.
>> > > > Current implementation may face a race condition.
>> > > > 
>> > > > For example, the potential race would be:
>> > > > 
>> > > >     CPU1                      CPU2
>> > > >     mem_cgroup_move_account   split_huge_page_to_list
>> > > >       !list_empty
>> > > >                                 lock
>> > > >                                 !list_empty
>> > > >                                 list_del
>> > > >                                 unlock
>> > > >       lock
>> > > >       # !list_empty might not hold anymore
>> > > >       list_del_init
>> > > >       unlock
>> > > 
>> > > I don't think this particular race is possible. Both parties take page
>> > > lock before messing with deferred queue, but anytway:
>> > > 
>> > > Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>> > 
>> > I am confused, if the above race is not possible then what would be a
>> > real race? We really do not want to have a patch with a misleading
>> > changelog, do we?
>> 
>> The alternative is to make sure that all page_deferred_list() called with
>> page lock taken.
>> 
>> I'll look into it.
>
>split_huge_page_to_list() has page lock taken.
>
>free_transhuge_page() is in the free path and doesn't susceptible to the
>race.
>
>deferred_split_scan() is trickier. list_move() should be safe against
>list_empty() as it will not produce false-positive list_empty().
>list_del_init() *should* (correct me if I'm wrong) be safe because the page
>is freeing and memcg will not touch the page anymore.
>
>deferred_split_huge_page() is a problematic one. It called from
>page_remove_rmap() path witch does require page lock. I don't see any
>obvious way to exclude race with mem_cgroup_move_account() here.
>Anybody else?

If my understanding is correct, the reason is deferred_split_huge_page()
doesn't has page lock taken, right?

>
>Wei, could you rewrite the commit message with deferred_split_huge_page()
>as a race source instead of split_huge_page_to_list()?
>
>-- 
> Kirill A. Shutemov

-- 
Wei Yang
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