Re: [PATCH v3] mm/hugetlb: Defer freeing of huge pages if in non-task context

[Waiman Long <longman@redhat.com>]

On 12/17/19 1:55 PM, Davidlohr Bueso wrote:
> On Tue, 17 Dec 2019, Waiman Long wrote:
>> Both the hugetbl_lock and the subpool lock can be acquired in
>> free_huge_page(). One way to solve the problem is to make both locks
>> irq-safe. However, Mike Kravetz had learned that the hugetlb_lock is
>> held for a linear scan of ALL hugetlb pages during a cgroup reparentling
>> operation. So it is just too long to have irq disabled unless we can
>> break hugetbl_lock down into finer-grained locks with shorter lock
>> hold times.
>>
>> Another alternative is to defer the freeing to a workqueue job.  This
>> patch implements the deferred freeing by adding a free_hpage_workfn()
>> work function to do the actual freeing. The free_huge_page() call in
>> a non-task context saves the page to be freed in the hpage_freelist
>> linked list in a lockless manner using the llist APIs.
>>
>> The generic workqueue is used to process the work, but a dedicated
>> workqueue can be used instead if it is desirable to have the huge page
>> freed ASAP.
>>
>> Thanks to Kirill Tkhai <ktkhai@virtuozzo.com> for suggesting the use
>> of llist APIs which simplfy the code.
>>
>> [v2: Add more comment & remove unneeded racing check]
>> [v3: Update commit log, remove pr_debug & use llist APIs]
>
> Very creative reusing the mapping pointer, along with the llist api,
> this solves the problem nicely (temporarily at least).
>
> Two small nits below.
>
> Acked-by: Davidlohr Bueso <dbueso@suse.de>
>
>> Reported-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>> mm/hugetlb.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++++++-
>> 1 file changed, 50 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> +static LLIST_HEAD(hpage_freelist);
>> +
>> +static void free_hpage_workfn(struct work_struct *work)
>> +{
>> +    struct llist_node *node;
>> +    struct page *page;
>> +
>> +    node = llist_del_all(&hpage_freelist);
>> +
>> +    while (node) {
>> +        page = container_of((struct address_space **)node,
>> +                     struct page, mapping);
>> +        node = node->next;
>
> llist_next()
I could use this helper, but the statement is simple enough to understand.
>
>> +        __free_huge_page(page);
>> +    }
>> +}
>> +static DECLARE_WORK(free_hpage_work, free_hpage_workfn);
>> +
>> +void free_huge_page(struct page *page)
>> +{
>> +    /*
>> +     * Defer freeing if in non-task context to avoid hugetlb_lock
>> deadlock.
>> +     */
>> +    if (!in_task()) {
>
> unlikely()?

Yes, I could use that too. For now, I am not going to post a v4 with
these changes unless that are other substantial changes that require a
respin.

Thanks,
Longman