Re: [PATCH v2 2/4] hugetlb: add HPageCma flag and code to free non-gigantic pages in CMA

[Mike Kravetz]

On 9/29/21 4:21 PM, Mike Kravetz wrote:
> On 9/29/21 12:42 PM, Mike Kravetz wrote:
>> On 9/24/21 2:36 AM, David Hildenbrand wrote:
>>> On 23.09.21 19:53, Mike Kravetz wrote:
>>>> When huge page demotion is fully implemented, gigantic pages can be
>>>> demoted to a smaller huge page size.  For example, on x86 a 1G page
>>>> can be demoted to 512 2M pages.  However, gigantic pages can potentially
>>>> be allocated from CMA.  If a gigantic page which was allocated from CMA
>>>> is demoted, the corresponding demoted pages needs to be returned to CMA.
>>>>
>>>> In order to track hugetlb pages that need to be returned to CMA, add the
>>>> hugetlb specific flag HPageCma.  Flag is set when a huge page is
>>>> allocated from CMA and transferred to any demoted pages.  Non-gigantic
>>>> huge page freeing code checks for the flag and takes appropriate action.
>>>
>>> Do we really need that flag or couldn't we simply always try cma_release() and fallback to out ordinary freeing-path?
>>>
>>> IIRC, cma knows exactly if something was allocated via a CMA are and can be free via it. No need for additional tracking usually.
>>>
>>
>> Yes, I think this is possible.
>> Initially, I thought the check for whether pages were part of CMA
>> involved a mutex or some type of locking.  But, it really is
>> lightweight.  So, should not be in issue calling in every case.
> 
> When modifying the code, I did come across one issue.  Sorry I did not
> immediately remember this.
> 
> Gigantic pages are allocated as a 'set of pages' and turned into a compound
> page by the hugetlb code.  They must be restored to a 'set of pages' before
> calling cma_release.  You can not pass a compound page to cma_release.
> Non-gigantic page are allocated from the buddy directly as compound pages.
> They are returned to buddy as a compound page.
> 
> So, the issue comes about when freeing a non-gigantic page.  We would
> need to convert to a 'set of pages' before calling cma_release just to
> see if cma_release succeeds.  Then, if it fails convert back to a
> compound page to call __free_pages.  Conversion is somewhat expensive as
> we must modify every tail page struct.
> 
> Some possible solutions:
> - Create a cma_pages_valid() routine that checks whether the pages
>   belong to a cma region.  Only convert to 'set of pages' if cma_pages_valid
>   and we know subsequent cma_release will succeed.
> - Always convert non-gigantic pages to a 'set of pages' before freeing.
>   Alternatively, don't allocate compound pages from buddy and just use
>   the hugetlb gigantic page prep and destroy routines for all hugetlb
>   page sizes.
> - Use some kind of flag as in proposed patch.
> 
> Having hugetlb just allocate a set of pages from buddy is interesting.
> This would make the allocate/free code paths for gigantic and
> non-gigantic pages align more closely.  It may in overall code simplification,
> not just for demote.

Taking this approach actually got a bit ugly in alloc_and_dissolve_huge_page
which is used in migration.  Instead, I took the approach of adding a
'cma_pages_valid()' interface to check at page freeing time.  Sending
out v3 shortly.
-- 
Mike Kravetz