Re: [PATCH v4 4/5] hugetlb: add demote bool to gigantic page routines

[Oscar Salvador]

On Fri, Oct 22, 2021 at 12:05:47PM -0700, Mike Kravetz wrote:
> How about if this commit message provides links to previous commits
> describing these issues?  There are pretty extensive descriptions in
> those previous commits, so no need to repeat here IMO.

Fine by me.

> The patch with an updated commit message is below.
> 
> From 10fcff70c809402901a93ea507d5506c87a8227d Mon Sep 17 00:00:00 2001
> From: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Date: Fri, 22 Oct 2021 11:50:31 -0700
> Subject: [PATCH v4 4/5] hugetlb: add demote bool to gigantic page routines
> 
> The routines remove_hugetlb_page and destroy_compound_gigantic_page
> will remove a gigantic page and make the set of base pages ready to be
> returned to a lower level allocator.  In the process of doing this, they
> make all base pages reference counted.
> 
> The routine prep_compound_gigantic_page creates a gigantic page from a
> set of base pages.  It assumes that all these base pages are reference
> counted.
> 
> During demotion, a gigantic page will be split into huge pages of a
> smaller size.  This logically involves use of the routines,
> remove_hugetlb_page, and destroy_compound_gigantic_page followed by
> prep_compound*_page for each smaller huge page.
> 
> When pages are reference counted (ref count >= 0), additional
> speculative ref counts could be taken as described in previous
> commits [1] and [2].  This could result in errors while demoting
> a huge page.  Quite a bit of code would need to be created to
> handle all possible issues.
> 
> Instead of dealing with the possibility of speculative ref counts, avoid
> the possibility by keeping ref counts at zero during the demote process.
> Add a boolean 'demote' to the routines remove_hugetlb_page,
> destroy_compound_gigantic_page and prep_compound_gigantic_page.  If the
> boolean is set, the remove and destroy routines will not reference count
> pages and the prep routine will not expect reference counted pages.
> 
> '*_for_demote' wrappers of the routines will be added in a subsequent
> patch where this functionality is used.
> 
> [1] https://lore.kernel.org/linux-mm/20210622021423.154662-3-mike.kravetz@xxxxxxxxxx/
> [2] https://lore.kernel.org/linux-mm/20210809184832.18342-3-mike.kravetz@xxxxxxxxxx/
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>

Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
 

-- 
Oscar Salvador
SUSE Labs