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

[Jerome Glisse]

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@xxxxxxxxxx>

Reviewed-by: Jérôme Glisse <jglisse@xxxxxxxxxx>

> ---
>  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
>