linux-kernel - Re: [PATCH v4] mm, thp: introduce generic transparent huge page allocation interfaces

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Date:   Fri, 8 Dec 2017 09:27:37 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     changbin.du@...el.com
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] mm, thp: introduce generic transparent huge page
 allocation interfaces

On Fri 08-12-17 12:42:55, changbin.du@...el.com wrote:
> From: Changbin Du <changbin.du@...el.com>
> 
> This patch introduced 4 new interfaces to allocate a prepared transparent
> huge page. These interfaces merge distributed two-step allocation as simple
> single step. And they can avoid issue like forget to call prep_transhuge_page()
> or call it on wrong page. A real fix:
> 40a899e ("mm: migrate: fix an incorrect call of prep_transhuge_page()")
> 
> Anyway, I just want to prove that expose direct allocation interfaces is
> better than a interface only do the second part of it.
> 
> These are similar to alloc_hugepage_xxx which are for hugetlbfs pages. New
> interfaces are:
>   - alloc_transhuge_page_vma
>   - alloc_transhuge_page_nodemask
>   - alloc_transhuge_page_node
>   - alloc_transhuge_page
> 
> These interfaces implicitly add __GFP_COMP gfp mask which is the minimum
> flags used for huge page allocation. More flags leave to the callers.
> 
> This patch does below changes:
>   - define alloc_transhuge_page_xxx interfaces
>   - apply them to all existing code
>   - declare prep_transhuge_page as static since no others use it
>   - remove alloc_hugepage_vma definition since it no longer has users

I am not really convinced this is a huge win, to be honest. Just look at
the diffstat. Very few callsites get marginally simpler while we add a
lot of stubs and the code churn.

> Signed-off-by: Changbin Du <changbin.du@...el.com>
> 
> ---
> v4:
>   - Revise the nop function definition. (Andrew)
> 
> v3:
>   - Rebase to latest mainline.
> 
> v2:
> Anshuman Khandu:
>   - Remove redundant 'VM_BUG_ON(!(gfp_mask & __GFP_COMP))'.
> Andrew Morton:
>   - Fix build error if thp is disabled.
> ---
>  include/linux/gfp.h     |  4 ----
>  include/linux/huge_mm.h | 35 +++++++++++++++++++++++++++++++++--
>  include/linux/migrate.h | 14 +++++---------
>  mm/huge_memory.c        | 48 +++++++++++++++++++++++++++++++++++++++++-------
>  mm/khugepaged.c         | 11 ++---------
>  mm/mempolicy.c          | 14 +++-----------
>  mm/migrate.c            | 14 ++++----------
>  mm/shmem.c              |  6 ++----
>  8 files changed, 90 insertions(+), 56 deletions(-)
-- 
Michal Hocko
SUSE Labs