Re: [External] Re: [PATCH v2 3/6] mm: hugetlb: fix a race between freeing and dissolving the page

[Mike Kravetz]

On 1/7/21 7:11 AM, Muchun Song wrote:
> On Thu, Jan 7, 2021 at 10:11 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>>
>> On Thu 07-01-21 20:59:33, Muchun Song wrote:
>>> On Thu, Jan 7, 2021 at 8:38 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>> [...]
>>>> Right. Can we simply back off in the dissolving path when ref count is
>>>> 0 && PageHuge() if list_empty(page->lru)? Is there any other scenario
>>>> when the all above is true and the page is not being freed?
>>>
>>> The list_empty(&page->lru) may always return false.
>>> The page before freeing is on the active list
>>> (hstate->hugepage_activelist).Then it is on the free list
>>> after freeing. So list_empty(&page->lru) is always false.
>>
>> The point I was trying to make is that the page has to be enqueued when
>> it is dissolved and freed. If the page is not enqueued then something
>> racing. But then I have realized that this is not a great check to
>> detect the race because pages are going to be released to buddy
>> allocator and that will reuse page->lru again. So scratch that and sorry
>> for the detour.
>>
>> But that made me think some more and one way to reliably detect the race
>> should be PageHuge() check in the freeing path. This is what dissolve
>> path does already. PageHuge becomes false during update_and_free_page()
>> while holding the hugetlb_lock. So can we use that?
> 
> It may make the thing complex. Apart from freeing it to the
> buddy allocator, free_huge_page also does something else for
> us. If we detect the race in the freeing path, if it is not a HugeTLB
> page, the freeing path just returns. We also should move those
> things to the dissolve path. Right?
> 
> But I find a tricky problem to solve. See free_huge_page().
> If we are in non-task context, we should schedule a work
> to free the page. We reuse the page->mapping. If the page
> is already freed by the dissolve path. We should not touch
> the page->mapping. So we need to check PageHuge().
> The check and llist_add() should be protected by
> hugetlb_lock. But we cannot do that. Right? If dissolve
> happens after it is linked to the list. We also should
> remove it from the list (hpage_freelist). It seems to make
> the thing more complex.

You are correct.  This is also an issue/potential problem with this
race.  It seems that adding the state information might be the least
complex way to address issue.

-- 
Mike Kravetz