linux-kernel - Re: [PATCH mm-unstable 8/8] mm/hugetlb: convert demote_free_huge

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Date:   Mon, 9 Jan 2023 10:36:44 -0600
From:   Sidhartha Kumar <sidhartha.kumar@...cle.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, songmuchun@...edance.com,
        tsahu@...ux.ibm.com, jhubbard@...dia.com
Subject: Re: [PATCH mm-unstable 8/8] mm/hugetlb: convert demote_free_huge_page
 to folios

On 1/7/23 2:55 PM, Mike Kravetz wrote:
> On 01/07/23 01:31, Matthew Wilcox wrote:
>> On Fri, Jan 06, 2023 at 05:11:36PM -0800, Mike Kravetz wrote:
>>> On 01/03/23 13:13, Sidhartha Kumar wrote:
>>>> @@ -3477,15 +3477,15 @@ static int demote_free_huge_page(struct hstate *h, struct page *page)
>>>>   	mutex_lock(&target_hstate->resize_lock);
>>>>   	for (i = 0; i < pages_per_huge_page(h);
>>>>   				i += pages_per_huge_page(target_hstate)) {
>>>> -		subpage = nth_page(page, i);
>>>> -		folio = page_folio(subpage);
>>>> +		subpage = folio_page(folio, i);
>>>> +		subfolio = page_folio(subpage);
>>>
>>> No problems with the code, but I am not in love with the name subfolio.
>>> I know it is patterned after 'subpage'.  For better or worse, the term
>>> subpage is used throughout the kernel.  This would be the first usage of
>>> the term 'subfolio'.
>>>
>>> Matthew do you have any comments on the naming?  It is local to hugetlb,
>>> but I would hate to see use of the term subfolio based on its introduction
>>> here.
>>
>> I'm really not a fan of it either.  I intended to dive into this patch
>> and understand the function it's modifying, in the hopes of suggesting
>> a better name and/or method.
> 
> At a high level, this routine is splitting a very large folio (1G for
> example) into multiple large folios of a smaller size (512 2M folios for
> example).  The loop is iterating through the very large folio at
> increments of the smaller large folio.  subfolio (previously subpage) is
> used to point to the smaller large folio within the loop.
> 
If folio does not need to be part of the variable name, how about 
something like 'demote_target'? The prep call inside the loop would then 
look like:

prep_new_hugetlb_folio(target_hstate, demote_target, nid);

so it is still clear that demote_target is a folio. A more concise 
version could also be 'demote_dst' but that seems more ambiguous than 
target.

Thanks,
Sidhartha Kumar