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Subject: Re: [PATCH v3 5/5] hugetlb: add hugetlb demote page support
To:     Oscar Salvador <osalvador@suse.de>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        David Hildenbrand <david@redhat.com>,
        Michal Hocko <mhocko@suse.com>, Zi Yan <ziy@nvidia.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Naoya Horiguchi <naoya.horiguchi@linux.dev>,
        David Rientjes <rientjes@google.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@linux.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>
References: <20211001175210.45968-1-mike.kravetz@oracle.com>
 <20211001175210.45968-6-mike.kravetz@oracle.com>
 <20211006084112.GA12288@linux>
From:   Mike Kravetz <mike.kravetz@oracle.com>
Message-ID: <1e699922-59a6-dc14-6676-b44c1bdf1a6f@oracle.com>
Date:   Wed, 6 Oct 2021 11:52:33 -0700
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On 10/6/21 1:41 AM, Oscar Salvador wrote:
> On Fri, Oct 01, 2021 at 10:52:10AM -0700, Mike Kravetz wrote:
>> Demote page functionality will split a huge page into a number of huge
>> pages of a smaller size.  For example, on x86 a 1GB huge page can be
>> demoted into 512 2M huge pages.  Demotion is done 'in place' by simply
>> splitting the huge page.
>>
>> Added '*_for_demote' wrappers for remove_hugetlb_page,
>> destroy_compound_gigantic_page and prep_compound_gigantic_page for use
>> by demote code.
>>
>> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
>> ---
>>  mm/hugetlb.c | 82 +++++++++++++++++++++++++++++++++++++++++++++++-----
>>  1 file changed, 74 insertions(+), 8 deletions(-)
>>
> ...  
>> +static int demote_free_huge_page(struct hstate *h, struct page *page)
>> +{
>> +	int i, nid = page_to_nid(page);
>> +	struct hstate *target_hstate;
>> +	int rc = 0;
>> +
>> +	target_hstate = size_to_hstate(PAGE_SIZE << h->demote_order);
>> +
>> +	remove_hugetlb_page_for_demote(h, page, false);
>> +	spin_unlock_irq(&hugetlb_lock);
>> +
>> +	rc = alloc_huge_page_vmemmap(h, page);
>> +	if (rc) {
>> +		/* Allocation of vmemmmap failed, we can not demote page */
>> +		spin_lock_irq(&hugetlb_lock);
>> +		set_page_refcounted(page);
>> +		add_hugetlb_page(h, page, false);
>> +		return rc;
>> +	}
> 
> Question: You keep the original error code returned from alloc_huge_page_vmemmap()
> here, but then you lose it on demote_pool_huge_page() when doing the
> !demote_free_huge_page. Would not make more sense to keep it all the way down to 
> demote_store() in case you want to return the actual error code?
> 

Yes, I will return it all the way to demote_store (and the user).

>> +
>> +	/*
>> +	 * Use destroy_compound_gigantic_page_for_demote for all huge page
>> +	 * sizes as it will not ref count pages.
>> +	 */
>> +	destroy_compound_gigantic_page_for_demote(page, huge_page_order(h));
> 
> It seems that for now we only allow gigantic pages to be demoted, but
> destroy_compound_gigantic_page_for_demote feels kind of wrong, even
> if it is only a wrapper that ends up calling _*gigantic_ functions.
> 
> We want a routine that destroy a hugetlb to be demoted into smaller hugetlb
> pages, so the name gigantic makes little sense to appear in my opinion.
> 

Agree, I do not love the name.  Since it is only a wrapper, how about
destroy_hugetlb_page_for_demote?  And, change those other *_for_demote
wrappers to similiarly not have gigantic in their names.

>>  static int demote_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed)
>>  	__must_hold(&hugetlb_lock)
>>  {
>> +	int nr_nodes, node;
>> +	struct page *page;
>>  	int rc = 0;
>>  
>>  	lockdep_assert_held(&hugetlb_lock);
>> @@ -3313,9 +3377,15 @@ static int demote_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed)
>>  	if (!h->demote_order)
>>  		return rc;
>>  
>> -	/*
>> -	 * TODO - demote fucntionality will be added in subsequent patch
>> -	 */
>> +	for_each_node_mask_to_free(h, nr_nodes, node, nodes_allowed) {
>> +		if (!list_empty(&h->hugepage_freelists[node])) {
>> +			page = list_entry(h->hugepage_freelists[node].next,
>> +					struct page, lru);
>> +			rc = !demote_free_huge_page(h, page);
> 
> I kinda dislike this as I pointed out.
> 

Will change.

Thanks for all your comments!
-- 
Mike Kravetz