Re: [PATCH v2 12/26] userfaultfd: wp: apply _PAGE_UFFD_WP bit

[Jerome Glisse <jglisse@redhat.com>]

On Tue, Feb 12, 2019 at 10:56:18AM +0800, Peter Xu wrote:
> Firstly, introduce two new flags MM_CP_UFFD_WP[_RESOLVE] for
> change_protection() when used with uffd-wp and make sure the two new
> flags are exclusively used.  Then,
> 
>   - For MM_CP_UFFD_WP: apply the _PAGE_UFFD_WP bit and remove _PAGE_RW
>     when a range of memory is write protected by uffd
> 
>   - For MM_CP_UFFD_WP_RESOLVE: remove the _PAGE_UFFD_WP bit and recover
>     _PAGE_RW when write protection is resolved from userspace
> 
> And use this new interface in mwriteprotect_range() to replace the old
> MM_CP_DIRTY_ACCT.
> 
> Do this change for both PTEs and huge PMDs.  Then we can start to
> identify which PTE/PMD is write protected by general (e.g., COW or soft
> dirty tracking), and which is for userfaultfd-wp.
> 
> Since we should keep the _PAGE_UFFD_WP when doing pte_modify(), add it
> into _PAGE_CHG_MASK as well.  Meanwhile, since we have this new bit, we
> can be even more strict when detecting uffd-wp page faults in either
> do_wp_page() or wp_huge_pmd().
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>

Few comments but still:

Reviewed-by: Jérôme Glisse <jglisse@redhat.com>

> ---
>  arch/x86/include/asm/pgtable_types.h |  2 +-
>  include/linux/mm.h                   |  5 +++++
>  mm/huge_memory.c                     | 14 +++++++++++++-
>  mm/memory.c                          |  4 ++--
>  mm/mprotect.c                        | 12 ++++++++++++
>  mm/userfaultfd.c                     |  8 ++++++--
>  6 files changed, 39 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index 8cebcff91e57..dd9c6295d610 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -133,7 +133,7 @@
>   */
>  #define _PAGE_CHG_MASK	(PTE_PFN_MASK | _PAGE_PCD | _PAGE_PWT |		\
>  			 _PAGE_SPECIAL | _PAGE_ACCESSED | _PAGE_DIRTY |	\
> -			 _PAGE_SOFT_DIRTY | _PAGE_DEVMAP)
> +			 _PAGE_SOFT_DIRTY | _PAGE_DEVMAP | _PAGE_UFFD_WP)
>  #define _HPAGE_CHG_MASK (_PAGE_CHG_MASK | _PAGE_PSE)

This chunk needs to be in the earlier arch specific patch.

[...]

> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 8d65b0f041f9..817335b443c2 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c

[...]

> @@ -2198,6 +2208,8 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  				entry = pte_mkold(entry);
>  			if (soft_dirty)
>  				entry = pte_mksoft_dirty(entry);
> +			if (uffd_wp)
> +				entry = pte_mkuffd_wp(entry);
>  		}
>  		pte = pte_offset_map(&_pmd, addr);
>  		BUG_ON(!pte_none(*pte));

Reading that code and i thought i would be nice if we could define a
pte_mask that we can or instead of all those if () entry |= ... but
that is just some dumb optimization and does not have any bearing on
the present patch. Just wanted to say that outloud.


> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index a6ba448c8565..9d4433044c21 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -46,6 +46,8 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>  	int target_node = NUMA_NO_NODE;
>  	bool dirty_accountable = cp_flags & MM_CP_DIRTY_ACCT;
>  	bool prot_numa = cp_flags & MM_CP_PROT_NUMA;
> +	bool uffd_wp = cp_flags & MM_CP_UFFD_WP;
> +	bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE;
>  
>  	/*
>  	 * Can be called with only the mmap_sem for reading by
> @@ -117,6 +119,14 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>  			if (preserve_write)
>  				ptent = pte_mk_savedwrite(ptent);
>  
> +			if (uffd_wp) {
> +				ptent = pte_wrprotect(ptent);
> +				ptent = pte_mkuffd_wp(ptent);
> +			} else if (uffd_wp_resolve) {
> +				ptent = pte_mkwrite(ptent);
> +				ptent = pte_clear_uffd_wp(ptent);
> +			}
> +
>  			/* Avoid taking write faults for known dirty pages */
>  			if (dirty_accountable && pte_dirty(ptent) &&
>  					(pte_soft_dirty(ptent) ||
> @@ -301,6 +311,8 @@ unsigned long change_protection(struct vm_area_struct *vma, unsigned long start,
>  {
>  	unsigned long pages;
>  
> +	BUG_ON((cp_flags & MM_CP_UFFD_WP_ALL) == MM_CP_UFFD_WP_ALL);

Don't you want to abort and return here if both flags are set ?

[...]