Re: [RFC PATCH 0/3] hugetlb: add demote/split page functionality

[Mike Kravetz]

On 3/9/21 1:01 AM, David Hildenbrand wrote:
> On 09.03.21 01:18, Mike Kravetz wrote:
>> To address these issues, introduce the concept of hugetlb page demotion.
>> Demotion provides a means of 'in place' splitting a hugetlb page to
>> pages of a smaller size.  For example, on x86 one 1G page can be
>> demoted to 512 2M pages.  Page demotion is controlled via sysfs files.
>> - demote_size    Read only target page size for demotion
>> - demote    Writable number of hugetlb pages to be demoted
>>
>> Only hugetlb pages which are free at the time of the request can be demoted.
>> Demotion does not add to the complexity surplus pages.  Demotion also honors
>> reserved huge pages.  Therefore, when a value is written to the sysfs demote
>> file that value is only the maximum number of pages which will be demoted.
>> It is possible fewer will actually be demoted.
>>
>> If demote_size is PAGESIZE, demote will simply free pages to the buddy
>> allocator.
> 
> With the vmemmap optimizations you will have to rework the vmemmap layout. How is that handled? Couldn't it happen that you are half-way through splitting a PUD into PMDs when you realize that you cannot allocate vmemmap pages for properly handling the remaining PMDs? What would happen then?
> 
> Or are you planning on making both features mutually exclusive?
> 
> Of course, one approach would be first completely restoring the vmemmap for the whole PUD (allocating more pages than necessary in the end) and then freeing individual pages again when optimizing the layout per PMD.
> 

You are right about the need to address this issue.  Patch 3 has the
comment:

+	/*
+	 * Note for future:
+	 * When support for reducing vmemmap of huge pages is added, we
+	 * will need to allocate vmemmap pages here and could fail.
+	 */

The simplest approach would be to restore the entire vmemmmap for the
larger page and then delete for smaller pages after the split.  We could
hook into the existing vmemmmap reduction code with just a few calls.
This would fail to demote/split, if the allocation fails.  However, this
is not optimal.

Ideally, the code would compute how many pages for vmemmmap are needed
after the split, allocate those and then construct vmmemmap
appropriately when creating the smaller pages.

I think we would want to always do the allocation of vmmemmap pages up
front and not even start the split process if the allocation fails.  No
sense starting something we may not be able to finish.

I purposely did not address that here as first I wanted to get feedback
on the usefulness demote functionality.
-- 
Mike Kravetz