linux-kernel - Re: [RFC v2 1/5] mm: defer THP insertion to khugepaged

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Message-ID: <c4d7c1e0-e83b-4666-8bfb-1220b59251c2@gmail.com>
Date: Mon, 17 Feb 2025 14:59:02 +0000
From: Usama Arif <usamaarif642@...il.com>
To: Nico Pache <npache@...hat.com>, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org,
 linux-mm@...ck.org
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Subject: Re: [RFC v2 1/5] mm: defer THP insertion to khugepaged



On 11/02/2025 00:40, Nico Pache wrote:
> setting /transparent_hugepages/enabled=always allows applications
> to benefit from THPs without having to madvise. However, the pf handler
> takes very few considerations to decide weather or not to actually use a
> THP. This can lead to a lot of wasted memory. khugepaged only operates
> on memory that was either allocated with enabled=always or MADV_HUGEPAGE.
> 
> Introduce the ability to set enabled=defer, which will prevent THPs from
> being allocated by the page fault handler unless madvise is set,
> leaving it up to khugepaged to decide which allocations will collapse to a
> THP. This should allow applications to benefits from THPs, while curbing
> some of the memory waste.
> 
> Signed-off-by: Nico Pache <npache@...hat.com>
> ---
>  include/linux/huge_mm.h | 15 +++++++++++++--
>  mm/huge_memory.c        | 31 +++++++++++++++++++++++++++----
>  2 files changed, 40 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 93e509b6c00e..fb381ca720ea 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -44,6 +44,7 @@ enum transparent_hugepage_flag {
>  	TRANSPARENT_HUGEPAGE_UNSUPPORTED,
>  	TRANSPARENT_HUGEPAGE_FLAG,
>  	TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
> +	TRANSPARENT_HUGEPAGE_DEFER_PF_INST_FLAG,

No strong preference, but maybe just TRANSPARENT_HUGEPAGE_DEFER_FLAG might be better?