Re: [mm-unstable PATCH v4 1/9] mm/hugetlb: check gigantic_page_runtime_supported() in return_unused_surplus_pages()

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/7/5 14:39, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Tue, Jul 05, 2022 at 10:16:39AM +0800, Miaohe Lin wrote:
>> On 2022/7/4 9:33, Naoya Horiguchi wrote:
>>> From: Naoya Horiguchi <naoya.horiguchi@nec.com>
>>>
>>> I found a weird state of 1GB hugepage pool, caused by the following
>>> procedure:
>>>
>>>   - run a process reserving all free 1GB hugepages,
>>>   - shrink free 1GB hugepage pool to zero (i.e. writing 0 to
>>>     /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages), then
>>>   - kill the reserving process.
>>>
>>> , then all the hugepages are free *and* surplus at the same time.
>>>
>>>   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
>>>   3
>>>   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/free_hugepages
>>>   3
>>>   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/resv_hugepages
>>>   0
>>>   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/surplus_hugepages
>>>   3
>>>
>>> This state is resolved by reserving and allocating the pages then
>>> freeing them again, so this seems not to result in serious problem.
>>> But it's a little surprising (shrinking pool suddenly fails).
>>>
>>> This behavior is caused by hstate_is_gigantic() check in
>>> return_unused_surplus_pages(). This was introduced so long ago in 2008
>>> by commit aa888a74977a ("hugetlb: support larger than MAX_ORDER"), and
>>> at that time the gigantic pages were not supposed to be allocated/freed
>>> at run-time.  Now kernel can support runtime allocation/free, so let's
>>> check gigantic_page_runtime_supported() together.
>>>
>>> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
>>
>> This patch looks good to me with a few question below.
> 
> Thank you for reviewing.
> 
>>
>>> ---
>>> v2 -> v3:
>>> - Fixed typo in patch description,
>>> - add !gigantic_page_runtime_supported() check instead of removing
>>>   hstate_is_gigantic() check (suggested by Miaohe and Muchun)
>>> - add a few more !gigantic_page_runtime_supported() check in
>>>   set_max_huge_pages() (by Mike).
>>> ---
>>>  mm/hugetlb.c | 19 ++++++++++++++++---
>>>  1 file changed, 16 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>> index 2a554f006255..bdc4499f324b 100644
>>> --- a/mm/hugetlb.c
>>> +++ b/mm/hugetlb.c
>>> @@ -2432,8 +2432,7 @@ static void return_unused_surplus_pages(struct hstate *h,
>>>  	/* Uncommit the reservation */
>>>  	h->resv_huge_pages -= unused_resv_pages;
>>>  
>>> -	/* Cannot return gigantic pages currently */
>>> -	if (hstate_is_gigantic(h))
>>> +	if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
>>>  		goto out;
>>>  
>>>  	/*
>>> @@ -3315,7 +3314,8 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
>>>  	 * the user tries to allocate gigantic pages but let the user free the
>>>  	 * boottime allocated gigantic pages.
>>>  	 */
>>> -	if (hstate_is_gigantic(h) && !IS_ENABLED(CONFIG_CONTIG_ALLOC)) {
>>> +	if (hstate_is_gigantic(h) && (!IS_ENABLED(CONFIG_CONTIG_ALLOC) ||
>>> +				      !gigantic_page_runtime_supported())) {
>>>  		if (count > persistent_huge_pages(h)) {
>>>  			spin_unlock_irq(&hugetlb_lock);
>>>  			mutex_unlock(&h->resize_lock);
>>> @@ -3363,6 +3363,19 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
>>>  			goto out;
>>>  	}
>>>  
>>> +	/*
>>> +	 * We can not decrease gigantic pool size if runtime modification
>>> +	 * is not supported.
>>> +	 */
>>> +	if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported()) {
>>> +		if (count < persistent_huge_pages(h)) {
>>> +			spin_unlock_irq(&hugetlb_lock);
>>> +			mutex_unlock(&h->resize_lock);
>>> +			NODEMASK_FREE(node_alloc_noretry);
>>> +			return -EINVAL;
>>> +		}
>>> +	}
>>
>> With above change, we're not allowed to decrease the pool size now. But it was allowed previously
>> even if !gigantic_page_runtime_supported. Does this will break user?
> 
> Yes, it does. I might get the wrong idea about the definition of
> gigantic_page_runtime_supported(), which shows that runtime pool *extension*
> is supported or not (implying that pool shrinking is always possible).
> If this is right, this new if-block is not necessary.

I tend to remove above new if-block to keep pool shrinking available.

Thanks.

> 
>>
>> And it seems it's not allowed to adjust the max_huge_pages now if !gigantic_page_runtime_supported
>> for gigantic huge page. Should we just return for such case as there should be nothing to do now?
>> Or am I miss something?
> 
> If pool shrinking is always allowed, we need uptdate max_huge_pages so,
> the above if-block should have "goto out;", but it will be removed anyway
> so we don't have to care for it.
> 
> Thank you for the valuable comment.
> 
> - Naoya Horiguchi
>