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

[Kirill A. Shutemov]

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@xxxxxxxxxxxxxxxxxxxx>
> CC: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> CC: Khalid Aziz <khalid.aziz@xxxxxxxxxx>
> CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> CC: "David S. Miller" <davem@xxxxxxxxxxxxx>
> CC: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> CC: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> CC: Henry Willard <henry.willard@xxxxxxxxxx>
> CC: Anshuman Khandual <khandual@xxxxxxxxxxxxxxxxxx>
> CC: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> CC: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
> CC: Jerome Glisse <jglisse@xxxxxxxxxx>
> CC: Zi Yan <zi.yan@xxxxxxxxxxxxxx>
> CC: linux-mm@xxxxxxxxx
> CC: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
> 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