linux-kernel - Re: [PATCH v2 1/2] mm: Make alloc_contig

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Date:   Fri, 19 Feb 2021 07:08:06 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Muchun Song <songmuchun@...edance.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm: Make alloc_contig_range handle free hugetlb
 pages

On 2021-02-19 03:10, Mike Kravetz wrote:
> Those counts will be wrong as there are no huge pages on the node.
> 
> I'll think about this more tomorrow.
> Pretty sure this is an issue, but I could be wrong.  Just wanted to 
> give
> a heads up.

Yes, this is a problem, although the fixup would be to check whether we 
have any hugepages.

Nevertheless, I think we should not be touching surplus at all but 
rather make the page temporary.
I am exploring making migrate_pages() handle free hugepages as Michal 
suggested, so the approach is cleaner and we do not need extra 
functions. I yet have to see if that is feasible, as some issues come to 
my mind like the page needs to be in a list to go to migrate_pages, but 
if it is in that list, it is not in hugepages_freelist, and that could 
disrupt userspace as it could not dequeue hugepages if it demands it.
I have to check. Shoult not be possible, we can always make the page 
temporary here.

> --
> Mike Kravetz
> 
>> +		}
>> +	}
>> +
>> +	return ret;
>> +}
>> +
>> +bool isolate_or_dissolve_huge_page(struct page *page)
>> +{
>> +	struct hstate *h = NULL;
>> +	struct page *head;
>> +	bool ret = false;
>> +
>> +	spin_lock(&hugetlb_lock);
>> +	if (PageHuge(page)) {
>> +		head = compound_head(page);
>> +		h = page_hstate(head);
>> +	}
>> +	spin_unlock(&hugetlb_lock);
>> +
>> +	/*
>> +	 * The page might have been dissolved from under our feet.
>> +	 * If that is the case, return success as if we dissolved it 
>> ourselves.
>> +	 */
>> +	if (!h)
>> +		return true;
>> +
>> +	/*
>> +	 * Fence off gigantic pages as there is a cyclic dependency
>> +	 * between alloc_contig_range and them.
>> +	 */
>> +	if (hstate_is_gigantic(h))
>> +		return ret;
>> +
>> +	if(!page_count(head) && alloc_and_dissolve_huge_page(h, head))
>> +		ret = true;
>> +
>> +	return ret;
>> +}
>> +
>>  struct page *alloc_huge_page(struct vm_area_struct *vma,
>>  				    unsigned long addr, int avoid_reserve)
>>  {
>> 

-- 
Oscar Salvador
SUSE L3