linux-kernel - Re: [PATCH 1/8] mm: move highest_order() and next

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Date: Wed, 8 May 2024 10:06:18 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
 hughd@...gle.com
Cc: willy@...radead.org, david@...hat.com, ioworker0@...il.com,
 wangkefeng.wang@...wei.com, ying.huang@...el.com, 21cnbao@...il.com,
 shy828301@...il.com, ziy@...dia.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/8] mm: move highest_order() and next_order() out of the
 THP config

On 08/05/2024 03:13, Baolin Wang wrote:
> 
> 
> On 2024/5/7 18:21, Ryan Roberts wrote:
>> On 06/05/2024 09:46, Baolin Wang wrote:
>>> Move highest_order() and next_order() out of the CONFIG_TRANSPARENT_HUGEPAGE
>>> macro, which can be common functions to be used.
>>
>> Sorry if I haven't kept up with the discussion, but why is this needed? I
>> wouldn't expect a need to iterate over orders if THP is compile-time disabled
>> because we will never try to allocate THP?
> 
> Cause I don't want to add some dummy functions to avoid building errors if
> CONFIG_TRANSPARENT_HUGEPAGE is not enabled in patch 6. Another thought is that
> the pagecache can also allocate a large folio even when THP is not enabled, so
> these helpers may be used in the future (not sure though).

OK, I'll admit I haven't looked at the latter patches yet - I'd like to conclude
on the interface and mapping/alignment strategy first.

But it wasn't necessary to access these functions for the anon/private case
without CONFIG_TRANSPARENT_HUGEPAGE, so I'm wondering why it's needed for shmem
case. I would expect that they don't need to be defined at all.

> 
> Anyway, I also have no strong perference for this patch, below dummy functions
> can also work for me:
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index c15bebb2cf53..7aa802ee2ce5 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -586,6 +586,16 @@ static inline bool thp_migration_supported(void)
>  {
>         return false;
>  }
> +
> +static inline int highest_order(unsigned long orders)
> +{
> +        return 0;
> +}
> +
> +static inline int next_order(unsigned long *orders, int prev)
> +{
> +        return 0;
> +}
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */