Re: [PATCH v2 18/26] khugepaged: skip collapse if uffd-wp detected

Mike Rapoport
2019-02-25T18:50:36Z

On Tue, Feb 12, 2019 at 10:56:24AM +0800, Peter Xu wrote:
> Don't collapse the huge PMD if there is any userfault write protected
> small PTEs. The problem is that the write protection is in small page
> granularity and there's no way to keep all these write protection
> information if the small pages are going to be merged into a huge PMD.
>
> The same thing needs to be considered for swap entries and migration
> entries. So do the check as well disregarding khugepaged_max_ptes_swap.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>

Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>

> ---
> include/trace/events/huge_memory.h | 1 +
> mm/khugepaged.c | 23 +++++++++++++++++++++++
> 2 files changed, 24 insertions(+)
>
> diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> index dd4db334bd63..2d7bad9cb976 100644
> --- a/include/trace/events/huge_memory.h
> +++ b/include/trace/events/huge_memory.h
> @@ -13,6 +13,7 @@
> EM( SCAN_PMD_NULL, "pmd_null") \
> EM( SCAN_EXCEED_NONE_PTE, "exceed_none_pte") \
> EM( SCAN_PTE_NON_PRESENT, "pte_non_present") \
> + EM( SCAN_PTE_UFFD_WP, "pte_uffd_wp") \
> EM( SCAN_PAGE_RO, "no_writable_page") \
> EM( SCAN_LACK_REFERENCED_PAGE, "lack_referenced_page") \
> EM( SCAN_PAGE_NULL, "page_null") \
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 4f017339ddb2..396c7e4da83e 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -29,6 +29,7 @@ enum scan_result {
> SCAN_PMD_NULL,
> SCAN_EXCEED_NONE_PTE,
> SCAN_PTE_NON_PRESENT,
> + SCAN_PTE_UFFD_WP,
> SCAN_PAGE_RO,
> SCAN_LACK_REFERENCED_PAGE,
> SCAN_PAGE_NULL,
> @@ -1123,6 +1124,15 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> pte_t pteval = *_pte;
> if (is_swap_pte(pteval)) {
> if (++unmapped <= khugepaged_max_ptes_swap) {
> + /*
> + * Always be strict with uffd-wp
> + * enabled swap entries. Please see
> + * comment below for pte_uffd_wp().
> + */
> + if (pte_swp_uffd_wp(pteval)) {
> + result = SCAN_PTE_UFFD_WP;
> + goto out_unmap;
> + }
> continue;
> } else {
> result = SCAN_EXCEED_SWAP_PTE;
> @@ -1142,6 +1152,19 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> result = SCAN_PTE_NON_PRESENT;
> goto out_unmap;
> }
> + if (pte_uffd_wp(pteval)) {
> + /*
> + * Don't collapse the page if any of the small
> + * PTEs are armed with uffd write protection.
> + * Here we can also mark the new huge pmd as
> + * write protected if any of the small ones is
> + * marked but that could bring uknown
> + * userfault messages that falls outside of
> + * the registered range. So, just be simple.
> + */
> + result = SCAN_PTE_UFFD_WP;
> + goto out_unmap;
> + }
> if (pte_write(pteval))
> writable = true;
>
> --
> 2.17.1
>

--
Sincerely yours,
Mike.