linux-kernel - Re: [RFC PATCH 3/8] mm/madvise: remove unnecessary checks on madvise_free_single

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Date:   Mon, 27 Sep 2021 12:17:09 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Peter Xu <peterx@...hat.com>,
        Nadav Amit <namit@...are.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Minchan Kim <minchan@...nel.org>,
        Colin Cross <ccross@...gle.com>,
        Suren Baghdasarya <surenb@...gle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 3/8] mm/madvise: remove unnecessary checks on
 madvise_free_single_vma()

On Sun, Sep 26, 2021 at 09:12:54AM -0700, Nadav Amit wrote:
> From: Nadav Amit <namit@...are.com>
> 
> madvise_free_single_vma() currently rechecks that the range fits within
> the VMA, adapts it accordingly, and returns -EINVAL if the range is
> entirely outside of the VMA.
> 
> The error-code of -EINVAL is incorrect according to the man pages (as it
> should have been -ENOMEM), but anyhow the range that is provided to
> madvise_free_single_vma() should always be valid. It is set correctly in
> do_madvise() and then rechecked in madvise_dontneed_free() is the
> mmap-lock is dropped.
> 
> Remove this check.
> 
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Minchan Kim <minchan@...nel.org>
> Cc: Colin Cross <ccross@...gle.com>
> Cc: Suren Baghdasarya <surenb@...gle.com>
> Cc: Mike Rapoport <rppt@...ux.vnet.ibm.com>
> Signed-off-by: Nadav Amit <namit@...are.com>
> ---
>  mm/madvise.c | 8 +-------
>  1 file changed, 1 insertion(+), 7 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index fe843513a4e8..17e39c70704b 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -716,14 +716,8 @@ static int madvise_free_single_vma(struct vm_area_struct *vma,
>  	if (!vma_is_anonymous(vma))
>  		return -EINVAL;
>  
> -	range.start = max(vma->vm_start, start_addr);
> -	if (range.start >= vma->vm_end)
> -		return -EINVAL;
> -	range.end = min(vma->vm_end, end_addr);
> -	if (range.end <= vma->vm_start)
> -		return -EINVAL;

How did you test this change?

As far as I can see, you leave 'range' uninitialized, but used for
walk_page_range() and mmu_notifiers.

NAK.

>  	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, mm,
> -				range.start, range.end);
> +				start_addr, end_addr);
>  
>  	lru_add_drain();
>  	tlb_gather_mmu(&tlb, mm);
> -- 
> 2.25.1
> 
> 

-- 
 Kirill A. Shutemov