Re: [PATCH] mm/page_alloc: Make alloc_gigantic_page() available for general use

[David Hildenbrand <david@redhat.com>]

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)


> [...]
>>> +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.

-- 

Thanks,

David / dhildenb