linux-kernel - Re: [PATCH v2] mm/mmap: Don't unlock VMAs in remap_file

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Message-ID: <413ffbe0-959d-c2ad-95d8-80104adac089@redhat.com>
Date:   Wed, 16 Dec 2020 15:58:39 +0100
From:   David Hildenbrand <david@...hat.com>
To:     "Liam R. Howlett" <Liam.Howlett@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...gle.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v2] mm/mmap: Don't unlock VMAs in remap_file_pages()

On 15.12.20 16:54, Liam R. Howlett wrote:
> do_mmap() will unlock the necessary VMAs.  There is also a bug in the
> loop which will evaluate as false and not unlock any VMAs anyways.

If there is a BUG, do we have a Fixes: tag? Also

1. Can we fix the bug separately first?
2. Can we have a better description on what the bug actually is
"evaluate as false"? What is the result of the bug?

CCing some people that might know if this is actually a sane change.
Skimming over do_mmap(), it's not immediately clear to me that
"do_mmap() will unlock the necessary VMAs".

> 
> Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> ---
>  mm/mmap.c | 18 +-----------------
>  1 file changed, 1 insertion(+), 17 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 5c8b4485860de..f7fecb77f84fd 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -3025,25 +3025,9 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
>  
>  	flags &= MAP_NONBLOCK;
>  	flags |= MAP_SHARED | MAP_FIXED | MAP_POPULATE;
> -	if (vma->vm_flags & VM_LOCKED) {
> -		struct vm_area_struct *tmp;
> +	if (vma->vm_flags & VM_LOCKED)
>  		flags |= MAP_LOCKED;
>  
> -		/* drop PG_Mlocked flag for over-mapped range */
> -		for (tmp = vma; tmp->vm_start >= start + size;
> -				tmp = tmp->vm_next) {
> -			/*
> -			 * Split pmd and munlock page on the border
> -			 * of the range.
> -			 */
> -			vma_adjust_trans_huge(tmp, start, start + size, 0);
> -
> -			munlock_vma_pages_range(tmp,
> -					max(tmp->vm_start, start),
> -					min(tmp->vm_end, start + size));
> -		}
> -	}
> -
>  	file = get_file(vma->vm_file);
>  	ret = do_mmap(vma->vm_file, start, size,
>  			prot, flags, pgoff, &populate, NULL);
> 


-- 
Thanks,

David / dhildenb