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Date:   Tue, 2 May 2023 12:15:25 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Lorenzo Stoakes <lstoakes@gmail.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        "Liam R . Howlett" <Liam.Howlett@oracle.com>,
        Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH] mm/mmap/vma_merge: always check invariants
Message-Id: <20230502121525.258f089cbed7e44c113f2d83@linux-foundation.org>
In-Reply-To: <df548a6ae3fa135eec3b446eb3dae8eb4227da97.1682885809.git.lstoakes@gmail.com>
References: <df548a6ae3fa135eec3b446eb3dae8eb4227da97.1682885809.git.lstoakes@gmail.com>
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On Sun, 30 Apr 2023 21:19:17 +0100 Lorenzo Stoakes <lstoakes@gmail.com> wrote:

> We may still have inconsistent input parameters even if we choose not to
> merge and the vma_merge() invariant checks are useful for checking this
> with no production runtime cost (these are only relevant when
> CONFIG_DEBUG_VM is specified).
> 
> Therefore, perform these checks regardless of whether we merge.
> 
> This is relevant, as a recent issue (addressed in commit "mm/mempolicy:
> Correctly update prev when policy is equal on mbind") in the mbind logic
> was only picked up in the 6.2.y stable branch where these assertions are
> performed prior to determining mergeability.
> 
> Had this remained the same in mainline this issue may have been picked up
> faster, so moving forward let's always check them.
> 

I'll scoot this into 6.4-rc, given the recent problems in this area.

> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -960,17 +960,17 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  		merge_next = true;
>  	}
>  
> +	/* Verify some invariant that must be enforced by the caller. */
> +	VM_WARN_ON(prev && addr <= prev->vm_start);
> +	VM_WARN_ON(curr && (addr != curr->vm_start || end > curr->vm_end));
> +	VM_WARN_ON(addr >= end);

Maybe converting to VM_WARN_ON_ONCE() would be kinder.