linux-kernel - Re: [PATCH v2] mm: mprotect: check page dirty when change ptes

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Date:   Wed, 12 Sep 2018 13:33:11 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "David S. Miller" <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Henry Willard <henry.willard@...cle.com>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Zi Yan <zi.yan@...rutgers.edu>, linux-mm@...ck.org
Subject: Re: [PATCH v2] mm: mprotect: check page dirty when change ptes

On Wed, Sep 12, 2018 at 02:49:21PM +0800, Peter Xu wrote:
> Add an extra check on page dirty bit in change_pte_range() since there
> might be case where PTE dirty bit is unset but it's actually dirtied.
> One example is when a huge PMD is splitted after written: the dirty bit
> will be set on the compound page however we won't have the dirty bit set
> on each of the small page PTEs.
> 
> I noticed this when debugging with a customized kernel that implemented
> userfaultfd write-protect.  In that case, the dirty bit will be critical
> since that's required for userspace to handle the write protect page
> fault (otherwise it'll get a SIGBUS with a loop of page faults).
> However it should still be good even for upstream Linux to cover more
> scenarios where we shouldn't need to do extra page faults on the small
> pages if the previous huge page is already written, so the dirty bit
> optimization path underneath can cover more.
> 
> CC: Andrew Morton <akpm@...ux-foundation.org>
> CC: Mel Gorman <mgorman@...hsingularity.net>
> CC: Khalid Aziz <khalid.aziz@...cle.com>
> CC: Thomas Gleixner <tglx@...utronix.de>
> CC: "David S. Miller" <davem@...emloft.net>
> CC: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> CC: Andi Kleen <ak@...ux.intel.com>
> CC: Henry Willard <henry.willard@...cle.com>
> CC: Anshuman Khandual <khandual@...ux.vnet.ibm.com>
> CC: Andrea Arcangeli <aarcange@...hat.com>
> CC: Kirill A. Shutemov <kirill@...temov.name>
> CC: Jerome Glisse <jglisse@...hat.com>
> CC: Zi Yan <zi.yan@...rutgers.edu>
> CC: linux-mm@...ck.org
> CC: linux-kernel@...r.kernel.org
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
> v2:
> - checking the dirty bit when changing PTE entries rather than fixing up
>   the dirty bit when splitting the huge page PMD.
> - rebase to 4.19-rc3
> 
> Instead of keeping this in my local tree, I'm giving it another shot to
> see whether this could be acceptable for upstream since IMHO it should
> still benefit the upstream.  Thanks,
> ---
>  mm/mprotect.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 6d331620b9e5..5fe752515161 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -115,6 +115,17 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>  			if (preserve_write)
>  				ptent = pte_mk_savedwrite(ptent);
>  
> +                       /*
> +                        * The extra PageDirty() check will make sure
> +                        * we'll capture the dirty page even if the PTE
> +                        * dirty bit is unset.  One case is when the
> +                        * PTE is splitted from a huge PMD, in that
> +                        * case the dirty flag might only be set on the
> +                        * compound page instead of this PTE.
> +                        */
> +			if (PageDirty(pte_page(ptent)))
> +				ptent = pte_mkdirty(ptent);
> +

How do you protect against concurent clearing of PG_dirty?

You can end up with unaccounted dirty page.

NAK.

-- 
 Kirill A. Shutemov