linux-kernel - Re: [PATCH v3 3/4] mm: prevent KSM from breaking VMA merging for new VMAs

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Message-Id: <20250622123931.30b1739642be8ec1e9ca04e2@linux-foundation.org>
Date: Sun, 22 Jun 2025 12:39:31 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner
 <brauner@...nel.org>, Jan Kara <jack@...e.cz>, "Liam R . Howlett"
 <Liam.Howlett@...cle.com>, Vlastimil Babka <vbabka@...e.cz>, Jann Horn
 <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>, David Hildenbrand
 <david@...hat.com>, Xu Xin <xu.xin16@....com.cn>, Chengming Zhou
 <chengming.zhou@...ux.dev>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, Stefan Roesch
 <shr@...kernel.io>
Subject: Re: [PATCH v3 3/4] mm: prevent KSM from breaking VMA merging for
 new VMAs

On Fri, 20 Jun 2025 13:48:09 +0100 Lorenzo Stoakes <lorenzo.stoakes@...cle.com> wrote:

> Hi Andrew,
> 
> Sending a fix-patch for this commit due to a reported syzbot issue which
> highlighted a bug in the implementation.
> 
> I discuss the syzbot report at [0].
> 
> [0]: https://lore.kernel.org/all/a55beb72-4288-4356-9642-76ab35a2a07c@lucifer.local/
> 
> There's a very minor conflict around the map->vm_flags vs. map->flags change,
> easily resolvable, but if you need a respin let me know.

I actually saw 4 conflicts, fixed various things up and...

> @@ -2487,6 +2496,11 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap)
>  	if (error)
>  		goto free_iter_vma;
> 
> +	if (!map->check_ksm_early) {
> +		update_ksm_flags(map);
> +		vm_flags_init(vma, map->vm_flags);
> +	}
> +

Guessing map->flags was intended here, I made that change then unmade
it in the later mm-update-core-kernel-code-to-use-vm_flags_t-consistently.patch.

I'll do a full rebuild at a couple of bisection points, please check
that all landed OK.