Re: WARNING: at mm/mremap.c:211 move_page_tables in i386

From: Joel Fernandes <joel@joelfernandes.org>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Naresh Kamboju <naresh.kamboju@linaro.org>,
	linux- stable <stable@vger.kernel.org>,
	open list <linux-kernel@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>, Arnd Bergmann <arnd@arndb.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Roman Gushchin <guro@fb.com>, Michal Hocko <mhocko@kernel.org>,
	lkft-triage@lists.linaro.org, Chris Down <chris@chrisdown.name>,
	Michel Lespinasse <walken@google.com>,
	Fan Yang <Fan_Yang@sjtu.edu.cn>,
	Brian Geffon <bgeffon@google.com>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Will Deacon <will@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	pugaowei@gmail.com, Jerome Glisse <jglisse@redhat.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Mel Gorman <mgorman@techsingularity.net>,
	Hugh Dickins <hughd@google.com>,
	Al Viro <viro@zeniv.linux.org.uk>, Tejun Heo <tj@kernel.org>,
	Sasha Levin <sashal@kernel.org>
Subject: Re: WARNING: at mm/mremap.c:211 move_page_tables in i386
Date: Tue, 14 Jul 2020 12:08:43 -0400	[thread overview]
Message-ID: <20200714160843.GA1685150@google.com> (raw)
In-Reply-To: <20200714073306.kq4zikkphqje2yzb@box>

Hi Kirill,

On Tue, Jul 14, 2020 at 10:33:06AM +0300, Kirill A. Shutemov wrote:
> On Sun, Jul 12, 2020 at 03:58:06PM -0700, Linus Torvalds wrote:
> > Anybody else have any opinions?
> 
> Maybe we just shouldn't allow move_normal_pmd() if ranges overlap?
> 
> Other option: pass 'overlaps' down to move_normal_pmd() and only WARN() if
> see establised PMD without overlaps being true.

I was thinking we should not call move_page_tables() with overlapping ranges
at all, just to keep things simple. I am concerned about other issues such as
if you move a range forward, you will end up overwriting part of the source
range.

Allow me some time to develop a proper patch, I have it on my list. I will
try to get to it this week.

I think we can also add a patch to detect the overlap as you did and warn in
such situation.

Thoughts?

thanks,

 - Joel

> Untested patch:
> 
> diff --git a/mm/mremap.c b/mm/mremap.c
> index 5dd572d57ca9..e33fcee541fe 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -245,6 +245,18 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
>  	unsigned long extent, next, old_end;
>  	struct mmu_notifier_range range;
>  	pmd_t *old_pmd, *new_pmd;
> +	bool overlaps;
> +
> +	/*
> +	 * shift_arg_pages() can call move_page_tables() on overlapping ranges.
> +	 * In this case we cannot use move_normal_pmd() because destination pmd
> +	 * might be established page table: move_ptes() doesn't free page
> +	 * table.
> +	 */
> +	if (old_addr > new_addr)
> +		overlaps = old_addr - new_addr < len;
> +	else
> +		overlaps = new_addr - old_addr < len;
>  
>  	old_end = old_addr + len;
>  	flush_cache_range(vma, old_addr, old_end);
> @@ -282,7 +294,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
>  			split_huge_pmd(vma, old_pmd, old_addr);
>  			if (pmd_trans_unstable(old_pmd))
>  				continue;
> -		} else if (extent == PMD_SIZE) {
> +		} else if (!overlaps && extent == PMD_SIZE) {
>  #ifdef CONFIG_HAVE_MOVE_PMD
>  			/*
>  			 * If the extent is PMD-sized, try to speed the move by
> -- 
>  Kirill A. Shutemov