linux-kernel - Re: [RFC 1/1] mm/madvise: fail MADV_PAGEOUT on VM

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Message-ID: <04d81e18-c206-40f3-b469-5ea588d4bf8e@lucifer.local>
Date: Mon, 20 Jan 2025 10:45:09 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, 21cnbao@...il.com, ryan.roberts@....com,
        dev.jain@....com, david@...hat.com, shy828301@...il.com,
        ziy@...dia.com, libang.li@...group.com, baolin.wang@...ux.alibaba.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Liam.Howlett@...cle.com, vbabka@...e.cz, jannh@...gle.com,
        Jason@...c4.com, Mingzhe Yang <mingzhe.yang@...com>
Subject: Re: [RFC 1/1] mm/madvise: fail MADV_PAGEOUT on VM_DROPPABLE VMA

Sorry but NACK again on this :(

On Mon, Jan 20, 2025 at 09:30:38AM +0800, Lance Yang wrote:
> MADV_PAGEOUT should fail on VMAs with the VM_DROPPABLE flag. While
> MADV_PAGEOUT is intended to move anonymous pages to swap, VM_DROPPABLE
> should not be swapped out.
>
> There is an issue where using MADV_PAGEOUT on a VMA with the VM_DROPPABLE
> flag behaves like MADV_DONTNEED, causing the pages to be dropped. This
> could break the semantics of MADV_PAGEOUT, IMO.
>
> So, let's add a check to detect the VM_DROPPABLE flag before doing
> MADV_PAGEOUT and returns -EINVAL.

No, let's not.

Firstly this behaviour, whether you feel it is right or not, is now
_established userland behaviour_. A pedantic interpretation of how
MADV_PAGEOUT ought to interact with MAP_DROPPABLE doesn't trump what is
already in released kernel versions, unless it is wrong in a -broken-
fashion.

Also I'd say you'd 100% expect an anon page to be dropped in the case of
MAP_DROPPABLE, the semantics of which are 'if there is a request to drop
this, drop it'. Here, the user is saying 'hey drop this'. So we drop it :)

I think the more correct patch would be to extend the man page to
explicitly mention MAP_DROPPABLE.

>
> Fixes: 9651fcedf7b9 ("mm: add MAP_DROPPABLE for designating always lazily freeable mappings")
> Signed-off-by: Mingzhe Yang <mingzhe.yang@...com>
> Signed-off-by: Lance Yang <ioworker0@...il.com>
> ---
>  mm/madvise.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 49f3a75046f6..29d0234da8a1 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1263,6 +1263,8 @@ static int madvise_vma_behavior(struct vm_area_struct *vma,
>  	case MADV_COLD:
>  		return madvise_cold(vma, prev, start, end);
>  	case MADV_PAGEOUT:
> +		if (vma->vm_flags & VM_DROPPABLE)
> +			return -EINVAL;
>  		return madvise_pageout(vma, prev, start, end);
>  	case MADV_FREE:
>  	case MADV_DONTNEED:
> --
> 2.45.2
>