linux-kernel - Re: [PATCH v4 2/9] mm: pagewalk: Take the pagetable lock in walk_pte

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Message-ID: <20191009151400.bserdtpoczmawqn5@box>
Date:   Wed, 9 Oct 2019 18:14:00 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        torvalds@...ux-foundation.org,
        Thomas Hellstrom <thellstrom@...are.com>,
        Matthew Wilcox <willy@...radead.org>,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rik van Riel <riel@...riel.com>,
        Minchan Kim <minchan@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        Huang Ying <ying.huang@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>
Subject: Re: [PATCH v4 2/9] mm: pagewalk: Take the pagetable lock in
 walk_pte_range()

On Tue, Oct 08, 2019 at 11:15:01AM +0200, Thomas Hellström (VMware) wrote:
> From: Thomas Hellstrom <thellstrom@...are.com>
> 
> Without the lock, anybody modifying a pte from within this function might
> have it concurrently modified by someone else.
> 
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Rik van Riel <riel@...riel.com>
> Cc: Minchan Kim <minchan@...nel.org>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Huang Ying <ying.huang@...el.com>
> Cc: Jérôme Glisse <jglisse@...hat.com>
> Cc: Kirill A. Shutemov <kirill@...temov.name>
> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Thomas Hellstrom <thellstrom@...are.com>
> ---
>  mm/pagewalk.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index d48c2a986ea3..83c0b78363b4 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -10,8 +10,9 @@ static int walk_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>  	pte_t *pte;
>  	int err = 0;
>  	const struct mm_walk_ops *ops = walk->ops;
> +	spinlock_t *ptl;
>  
> -	pte = pte_offset_map(pmd, addr);
> +	pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
>  	for (;;) {
>  		err = ops->pte_entry(pte, addr, addr + PAGE_SIZE, walk);
>  		if (err)
> @@ -22,7 +23,7 @@ static int walk_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>  		pte++;
>  	}
>  
> -	pte_unmap(pte);
> +	pte_unmap_unlock(pte - 1, ptl);

NAK.

If ->pte_entry() fails on the first entry of the page table, pte - 1 will
point out side the page table.

And the '- 1' is totally unnecessary as we break the loop before pte++ on
the last iteration.

-- 
 Kirill A. Shutemov