linux-kernel - Re: [PATCH v2 3/3] mm/vma: remove mmap() retry merge

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Message-ID: <bfd5b4co3ha32m7z7fo7rm2uhaqcunn7zqgywonm54wi2iakpb@zogdqwo4722c>
Date: Tue, 13 May 2025 09:25:36 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>, Vlastimil Babka <vbabka@...e.cz>,
        Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v2 3/3] mm/vma: remove mmap() retry merge

* Lorenzo Stoakes <lorenzo.stoakes@...cle.com> [250509 08:14]:
> We have now introduced a mechanism that obviates the need for a reattempted
> merge via the mmap_prepare() file hook, so eliminate this functionality
> altogether.
> 
> The retry merge logic has been the cause of a great deal of complexity in
> the past and required a great deal of careful manoeuvring of code to ensure
> its continued and correct functionality.
> 
> It has also recently been involved in an issue surrounding maple tree
> state, which again points to its problematic nature.
> 
> We make it much easier to reason about mmap() logic by eliminating this and
> simply writing a VMA once. This also opens the doors to future optimisation
> and improvement in the mmap() logic.
> 
> For any device or file system which encounters unwanted VMA fragmentation
> as a result of this change (that is, having not implemented .mmap_prepare
> hooks), the issue is easily resolvable by doing so.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Reviewed-by: David Hildenbrand <david@...hat.com>

I have a few tests for the vma test suite that would test this path.
I'll just let them go.

Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>

> ---
>  mm/vma.c | 14 --------------
>  1 file changed, 14 deletions(-)
> 
> diff --git a/mm/vma.c b/mm/vma.c
> index 3f32e04bb6cc..3ff6cfbe3338 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -24,7 +24,6 @@ struct mmap_state {
>  	void *vm_private_data;
>  
>  	unsigned long charged;
> -	bool retry_merge;
>  
>  	struct vm_area_struct *prev;
>  	struct vm_area_struct *next;
> @@ -2417,8 +2416,6 @@ static int __mmap_new_file_vma(struct mmap_state *map,
>  			!(map->flags & VM_MAYWRITE) &&
>  			(vma->vm_flags & VM_MAYWRITE));
>  
> -	/* If the flags change (and are mergeable), let's retry later. */
> -	map->retry_merge = vma->vm_flags != map->flags && !(vma->vm_flags & VM_SPECIAL);
>  	map->flags = vma->vm_flags;
>  
>  	return 0;
> @@ -2622,17 +2619,6 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr,
>  	if (have_mmap_prepare)
>  		set_vma_user_defined_fields(vma, &map);
>  
> -	/* If flags changed, we might be able to merge, so try again. */
> -	if (map.retry_merge) {
> -		struct vm_area_struct *merged;
> -		VMG_MMAP_STATE(vmg, &map, vma);
> -
> -		vma_iter_config(map.vmi, map.addr, map.end);
> -		merged = vma_merge_existing_range(&vmg);
> -		if (merged)
> -			vma = merged;
> -	}
> -
>  	__mmap_complete(&map, vma);
>  
>  	return addr;
> -- 
> 2.49.0
>