linux-kernel - Re: [PATCH v2 2/2] mm: shmem: disallow hugepages if the system-wide shmem THP sysfs settings are disabled

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Message-ID: <b6ae32e5-60e0-44dd-a1e8-37c162d04ed3@lucifer.local>
Date: Sat, 7 Jun 2025 13:14:41 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, hughd@...gle.com, david@...hat.com,
        Liam.Howlett@...cle.com, npache@...hat.com, ryan.roberts@....com,
        dev.jain@....com, ziy@...dia.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] mm: shmem: disallow hugepages if the system-wide
 shmem THP sysfs settings are disabled

On Thu, Jun 05, 2025 at 04:00:59PM +0800, Baolin Wang wrote:
> The MADV_COLLAPSE will ignore the system-wide shmem THP sysfs settings, which
> means that even though we have disabled the shmem THP configuration, MADV_COLLAPSE
> will still attempt to collapse into a shmem THP. This violates the rule we have
> agreed upon: never means never.

Ugh that we have separate shmem logic. And split between huge_memory.c and
shmem.c too :)) Again, not your fault, just a general moan about existing
stuff :P

>
> Another rule for madvise, referring to David's suggestion: “allowing for collapsing
> in a VM without VM_HUGEPAGE in the "madvise" mode would be fine".

Hm I'm not sure if this is enforced is it? I may have missed something here
however.

>
> Then the current strategy is:
>
> For shmem, if none of always, madvise, within_size, and inherit have enabled
> PMD-sized THP, then MADV_COLLAPSE will be prohibited from collapsing PMD-sized THP.

Again, is this just MADV_COLLAPSE? Surely this is a general change?

We should be clear that we are not explicitly limiting ourselves to
MADV_COLLAPSE here.

You shoudl clearly indicate that the MADV_COLLAPSE case DOESN'T set
TVA_ENFORCE_SYSFS and that's the key difference here.

>
> For tmpfs, if the mount option is set with the 'huge=never' parameter, then
> MADV_COLLAPSE will be prohibited from collapsing PMD-sized THP.
>
> Acked-by: Zi Yan <ziy@...dia.com>
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> ---
>  mm/huge_memory.c | 2 +-
>  mm/shmem.c       | 6 +++---
>  2 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index d3e66136e41a..a8cfa37cae72 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -166,7 +166,7 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>  	 * own flags.
>  	 */
>  	if (!in_pf && shmem_file(vma->vm_file))
> -		return shmem_allowable_huge_orders(file_inode(vma->vm_file),
> +		return orders & shmem_allowable_huge_orders(file_inode(vma->vm_file),
>  						   vma, vma->vm_pgoff, 0,
>  						   !enforce_sysfs);

Did you mean to do &&?

Also, is this achieving what you want to achieve? Is it necessary? The
changes in patch 1/2 enforce the global settings and before this code in
__thp_vma_allowable_orders():

unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
					 unsigned long vm_flags,
					 unsigned long tva_flags,
					 unsigned long orders)
{
	... (no early exits) ...

	orders &= supported_orders;
	if (!orders)
		return 0;

	...
}

So if orders == 0 due to the changes in thp_vma_allowable_orders(), which
is the only caller of __thp_vma_allowable_orders() then we _always_ exit
early here before we get to this shmem_allowable_huge_orders() code.

>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 4b42419ce6b2..9af45d4e27e6 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -625,7 +625,7 @@ static unsigned int shmem_huge_global_enabled(struct inode *inode, pgoff_t index
>  		return 0;
>  	if (shmem_huge == SHMEM_HUGE_DENY)
>  		return 0;
> -	if (shmem_huge_force || shmem_huge == SHMEM_HUGE_FORCE)
> +	if (shmem_huge == SHMEM_HUGE_FORCE)
>  		return maybe_pmd_order;
>
>  	/*
> @@ -660,7 +660,7 @@ static unsigned int shmem_huge_global_enabled(struct inode *inode, pgoff_t index
>
>  		fallthrough;
>  	case SHMEM_HUGE_ADVISE:
> -		if (vm_flags & VM_HUGEPAGE)
> +		if (shmem_huge_force || (vm_flags & VM_HUGEPAGE))
>  			return maybe_pmd_order;
>  		fallthrough;
>  	default:
> @@ -1790,7 +1790,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
>  	/* Allow mTHP that will be fully within i_size. */
>  	mask |= shmem_get_orders_within_size(inode, within_size_orders, index, 0);
>
> -	if (vm_flags & VM_HUGEPAGE)
> +	if (shmem_huge_force || (vm_flags & VM_HUGEPAGE))
>  		mask |= READ_ONCE(huge_shmem_orders_madvise);

I'm also not sure these are necessary:

The only path that can set shmem_huge_force is __thp_vma_allowable_orders()
-> shmem_allowable_huge_orders() -> shmem_huge_global_enabled() and then
only if !(tva_flags & TVA_ENFORCE_SYSFS) and as stated above we already
cover off this case by early exiting __thp_vma_allowable_orders() if orders
== 0 as established in patch 1/2.



>
>  	if (global_orders > 0)
> --
> 2.43.5
>