linux-kernel - Re: [PATCH] mm/page_alloc: Make alloc_gigantic

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Date:   Wed, 16 Oct 2019 13:10:41 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Anshuman Khandual <anshuman.khandual@....com>, linux-mm@...ck.org,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Rientjes <rientjes@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Pavel Tatashin <pavel.tatashin@...rosoft.com>,
        Matthew Wilcox <willy@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/page_alloc: Make alloc_gigantic_page() available for
 general use

On 16.10.19 13:08, Michal Hocko wrote:
> On Wed 16-10-19 10:56:16, David Hildenbrand wrote:
>> On 16.10.19 10:51, Michal Hocko wrote:
>>> On Wed 16-10-19 10:08:21, David Hildenbrand wrote:
>>>> On 16.10.19 09:34, Anshuman Khandual wrote:
>>> [...]
>>>>> +static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,
>>>>> +				   unsigned long nr_pages)
>>>>> +{
>>>>> +	unsigned long i, end_pfn = start_pfn + nr_pages;
>>>>> +	struct page *page;
>>>>> +
>>>>> +	for (i = start_pfn; i < end_pfn; i++) {
>>>>> +		page = pfn_to_online_page(i);
>>>>> +		if (!page)
>>>>> +			return false;
>>>>> +
>>>>> +		if (page_zone(page) != z)
>>>>> +			return false;
>>>>> +
>>>>> +		if (PageReserved(page))
>>>>> +			return false;
>>>>> +
>>>>> +		if (page_count(page) > 0)
>>>>> +			return false;
>>>>> +
>>>>> +		if (PageHuge(page))
>>>>> +			return false;
>>>>> +	}
>>>>
>>>> We might still try to allocate a lot of ranges that contain unmovable data
>>>> (we could avoid isolating a lot of page blocks in the first place). I'd love
>>>> to see something like pfn_range_movable() (similar, but different to
>>>> is_mem_section_removable(), which uses has_unmovable_pages()).
>>>
>>> Just to make sure I understand. Do you want has_unmovable_pages to be
>>> called inside pfn_range_valid_contig?
>>
>> I think this requires more thought, as has_unmovable_pages() works on
>> pageblocks only AFAIK. If you try to allocate < MAX_ORDER - 1, you could get
>> a lot of false positives.
>>
>> E.g., if a free "MAX_ORDER - 1" page spans two pageblocks and you only test
>> the second pageblock, you might detect "unmovable" if not taking proper care
>> of the "bigger" free page. (alloc_contig_range() properly works around that
>> issue)
> 
> OK, I see your point. You are right that false positives are possible. I
> would deal with those in a separate patch though.
> 
>>> [...]
>>>>> +struct page *alloc_contig_pages(unsigned long nr_pages, gfp_t gfp_mask,
>>>>> +				int nid, nodemask_t *nodemask)
>>>>> +{
>>>>> +	unsigned long ret, pfn, flags;
>>>>> +	struct zonelist *zonelist;
>>>>> +	struct zone *zone;
>>>>> +	struct zoneref *z;
>>>>> +
>>>>> +	zonelist = node_zonelist(nid, gfp_mask);
>>>>> +	for_each_zone_zonelist_nodemask(zone, z, zonelist,
>>>>> +					gfp_zone(gfp_mask), nodemask) {
>>>>
>>>> One important part is to never use the MOVABLE zone here (otherwise
>>>> unmovable data would end up on the movable zone). But I guess the caller is
>>>> responsible for that (not pass GFP_MOVABLE) like gigantic pages do.
>>>
>>> Well, if the caller uses GFP_MOVABLE then the movability should be
>>> implemented in some form. If that is not the case then it is a bug on
>>> the caller behalf.
>>>
>>>>> +		spin_lock_irqsave(&zone->lock, flags);
>>>>> +
>>>>> +		pfn = ALIGN(zone->zone_start_pfn, nr_pages);
>>>>
>>>> This alignment does not make too much sense when allowing passing in !power
>>>> of two orders. Maybe the caller should specify the requested alignment
>>>> instead? Or should we enforce this to be aligned to make our life easier for
>>>> now?
>>>
>>> Are there any usecases that would require than the page alignment?
>>
>> Gigantic pages have to be aligned AFAIK.
> 
> Aligned to what? I do not see any guarantee like that in the existing
> code.
> 

pfn = ALIGN(zone->zone_start_pfn, nr_pages);

-- 

Thanks,

David / dhildenb