On 4/2/21 10:59 PM, Muchun Song wrote:
> On Sat, Apr 3, 2021 at 4:56 AM Mike Kravetz <mike.krav...@oracle.com> wrote:
>>
>> On 4/2/21 5:47 AM, Muchun Song wrote:
>>> On Wed, Mar 31, 2021 at 11:42 AM Mike Kravetz <mike.krav...@oracle.com>
>>> wrote:
>>>>
>>>> Commit c77c0a8ac4c5 ("mm/hugetlb: defer freeing of huge pages if in
>>>> non-task context") was added to address the issue of free_huge_page
>>>> being called from irq context. That commit hands off free_huge_page
>>>> processing to a workqueue if !in_task. However, this doesn't cover
>>>> all the cases as pointed out by 0day bot lockdep report [1].
>>>>
>>>> : Possible interrupt unsafe locking scenario:
>>>> :
>>>> : CPU0 CPU1
>>>> : ---- ----
>>>> : lock(hugetlb_lock);
>>>> : local_irq_disable();
>>>> : lock(slock-AF_INET);
>>>> : lock(hugetlb_lock);
>>>> : <Interrupt>
>>>> : lock(slock-AF_INET);
>>>>
>>>> Shakeel has later explained that this is very likely TCP TX zerocopy
>>>> from hugetlb pages scenario when the networking code drops a last
>>>> reference to hugetlb page while having IRQ disabled. Hugetlb freeing
>>>> path doesn't disable IRQ while holding hugetlb_lock so a lock dependency
>>>> chain can lead to a deadlock.
>>>>
>>>> This commit addresses the issue by doing the following:
>>>> - Make hugetlb_lock irq safe. This is mostly a simple process of
>>>> changing spin_*lock calls to spin_*lock_irq* calls.
>>>> - Make subpool lock irq safe in a similar manner.
>>>> - Revert the !in_task check and workqueue handoff.
>>>>
>>>> [1]
>>>> https://lore.kernel.org/linux-mm/000000000000f1c03b05bc43a...@google.com/
>>>>
>>>> Signed-off-by: Mike Kravetz <mike.krav...@oracle.com>
>>>> Acked-by: Michal Hocko <mho...@suse.com>
>>>> Reviewed-by: Muchun Song <songmuc...@bytedance.com>
>>>
>>> Hi Mike,
>>>
>>> Today I pulled the newest code (next-20210401). I found that
>>> alloc_and_dissolve_huge_page is not updated. In this function,
>>> hugetlb_lock is still non-irq safe. Maybe you or Oscar need
>>> to fix.
>>>
>>> Thanks.
>>
>> Thank you Muchun,
>>
>> Oscar's changes were not in Andrew's tree when I started on this series
>> and I failed to notice their inclusion. In addition,
>> isolate_or_dissolve_huge_page also needs updating as well as a change in
>> set_max_huge_pages that was omitted while rebasing.
>>
>> Andrew, the following patch addresses those missing changes. Ideally,
>> the changes should be combined/included in this patch. If you want me
>> to sent another version of this patch or another series, let me know.
>>
>> From 450593eb3cea895f499ddc343c22424c552ea502 Mon Sep 17 00:00:00 2001
>> From: Mike Kravetz <mike.krav...@oracle.com>
>> Date: Fri, 2 Apr 2021 13:18:13 -0700
>> Subject: [PATCH] hugetlb: fix irq locking omissions
>>
>> The pach "hugetlb: make free_huge_page irq safe" changed spin_*lock
>> calls to spin_*lock_irq* calls. However, it missed several places
>> in the file hugetlb.c. Add the overlooked changes.
>>
>> Signed-off-by: Mike Kravetz <mike.krav...@oracle.com>
>
> Thanks MIke. But there are still some places that need
> improvement. See below.
>
Correct. My apologies again for not fully taking into account the new
code from Oscar's series when working on this.
>> ---
>> mm/hugetlb.c | 16 ++++++++--------
>> 1 file changed, 8 insertions(+), 8 deletions(-)
>>
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index c22111f3da20..a6bfc6bcbc81 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -2284,7 +2284,7 @@ static int alloc_and_dissolve_huge_page(struct hstate
>> *h, struct page *old_page,
>> */
>> page_ref_dec(new_page);
>> retry:
>> - spin_lock(&hugetlb_lock);
>> + spin_lock_irq(&hugetlb_lock);
>> if (!PageHuge(old_page)) {
>> /*
>> * Freed from under us. Drop new_page too.
>> @@ -2297,7 +2297,7 @@ static int alloc_and_dissolve_huge_page(struct hstate
>> *h, struct page *old_page,
>> * Fail with -EBUSY if not possible.
>> */
>> update_and_free_page(h, new_page);
>
> Now update_and_free_page can be called without holding
> hugetlb_lock. We can move it out of hugetlb_lock. In this
> function, there are 3 places which call update_and_free_page().
> We can move all of them out of hugetlb_lock. Right?
We will need to do more than that.
The call to update_and_free_page in alloc_and_dissolve_huge_page
assumes old functionality. i.e. It assumes h->nr_huge_pages will be
decremented in update_and_free_page. This is no longer the case.
This will need to be fixed in patch 4 of my series which changes the
functionality of update_and_free_page. I'm afraid a change there will
end up requiring changes in subsequent patches due to context.
I will have an update on Monday.
--
Mike Kravetz
