linux-kernel - Re: [PATCH v6 1/4] mm: defer THP insertion to khugepaged

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Message-ID: <fcd3phzewpgzghrzse3stxi7jz7b6l5uwnhqtswcfnqvuvktip@apqh2achkutn>
Date: Sat, 14 Jun 2025 13:25:27 +0200
From: Klara Modin <klarasmodin@...il.com>
To: Nico Pache <npache@...hat.com>
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Subject: Re: [PATCH v6 1/4] mm: defer THP insertion to khugepaged

Hi,

On 2025-05-14 21:38:54 -0600, Nico Pache wrote:
> setting /transparent_hugepages/enabled=always allows applications
> to benefit from THPs without having to madvise. However, the page fault
> 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.
> 
> Acked-by: Zi Yan <ziy@...dia.com>
> Co-developed-by: Rafael Aquini <raquini@...hat.com>
> Signed-off-by: Rafael Aquini <raquini@...hat.com>
> Signed-off-by: Nico Pache <npache@...hat.com>

...

> @@ -315,13 +318,20 @@ static ssize_t enabled_store(struct kobject *kobj,
>  
>  	if (sysfs_streq(buf, "always")) {
>  		clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, &transparent_hugepage_flags);
> +		clear_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_FLAG, &transparent_hugepage_flags);
>  		set_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags);
> +	} else if (sysfs_streq(buf, "defer")) {
> +		clear_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags);
> +		clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, &transparent_hugepage_flags);
> +		set_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_FLAG, &transparent_hugepage_flags);
>  	} else if (sysfs_streq(buf, "madvise")) {
>  		clear_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags);
> +		clear_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_FLAG, &transparent_hugepage_flags);
>  		set_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, &transparent_hugepage_flags);
>  	} else if (sysfs_streq(buf, "never")) {
>  		clear_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags);
>  		clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, &transparent_hugepage_flags);
> +		clear_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_FLAG, &transparent_hugepage_flags);
>  	} else
>  		ret = -EINVAL;
>  
> @@ -954,18 +964,31 @@ static int __init setup_transparent_hugepage(char *str)
>  			&transparent_hugepage_flags);
>  		clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
>  			  &transparent_hugepage_flags);
> +		clear_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_FLAG,
> +			  &transparent_hugepage_flags);
>  		ret = 1;
> +	} else if (!strcmp(str, "defer")) {
> +		clear_bit(TRANSPARENT_HUGEPAGE_FLAG,
> +			  &transparent_hugepage_flags);
> +		clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
> +			  &transparent_hugepage_flags);
> +		set_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_FLAG,
> +			  &transparent_hugepage_flags);

There should probably be a corresponding
		ret = 1;
here. Otherwise the cannot parse message will displayed even if defer
was set.

>  	} else if (!strcmp(str, "madvise")) {
>  		clear_bit(TRANSPARENT_HUGEPAGE_FLAG,
>  			  &transparent_hugepage_flags);
> +		clear_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_FLAG,
> +			  &transparent_hugepage_flags);
>  		set_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
> -			&transparent_hugepage_flags);
> +			  &transparent_hugepage_flags);
>  		ret = 1;
>  	} else if (!strcmp(str, "never")) {
>  		clear_bit(TRANSPARENT_HUGEPAGE_FLAG,
>  			  &transparent_hugepage_flags);
>  		clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
>  			  &transparent_hugepage_flags);
> +		clear_bit(TRANSPARENT_HUGEPAGE_DEFER_PF_FLAG,
> +			  &transparent_hugepage_flags);
>  		ret = 1;
>  	}
>  out:
> -- 
> 2.49.0
> 

Regards,
Klara Modin