Re: [PATCH v7 1/3] mm/mmap: separate writenotify and dirty tracking logic

[David Hildenbrand]

On 02.05.23 18:34, Lorenzo Stoakes wrote:
> vma_wants_writenotify() is specifically intended for setting PTE page table
> flags, accounting for existing PTE flag state and whether that might
> already be read-only while mixing this check with a check whether the
> filesystem performs dirty tracking.
>
> Separate out the notions of dirty tracking and a PTE write notify checking
> in order that we can invoke the dirty tracking check from elsewhere.
>
> Note that this change introduces a very small duplicate check of the
> separated out vm_ops_needs_writenotify(). This is necessary to avoid making
> vma_needs_dirty_tracking() needlessly complicated (e.g. passing a
> check_writenotify flag or having it assume this check was already
> performed). This is such a small check that it doesn't seem too egregious
> to do this.
>
> Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
> Reviewed-by: Mika Penttilä <mpenttil@xxxxxxxxxx>
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> ---
>   include/linux/mm.h |  1 +
>   mm/mmap.c          | 36 +++++++++++++++++++++++++++---------
>   2 files changed, 28 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 27ce77080c79..7b1d4e7393ef 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2422,6 +2422,7 @@ extern unsigned long move_page_tables(struct vm_area_struct *vma,
>   #define  MM_CP_UFFD_WP_ALL                 (MM_CP_UFFD_WP | \
>   					    MM_CP_UFFD_WP_RESOLVE)
>   
> +bool vma_needs_dirty_tracking(struct vm_area_struct *vma);
>   int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot);
>   static inline bool vma_wants_manual_pte_write_upgrade(struct vm_area_struct *vma)
>   {
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 5522130ae606..295c5f2e9bd9 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1475,6 +1475,31 @@ SYSCALL_DEFINE1(old_mmap, struct mmap_arg_struct __user *, arg)
>   }
>   #endif /* __ARCH_WANT_SYS_OLD_MMAP */
>   
> +/* Do VMA operations imply write notify is required? */
> +static bool vm_ops_needs_writenotify(const struct vm_operations_struct *vm_ops)
> +{
> +	return vm_ops && (vm_ops->page_mkwrite || vm_ops->pfn_mkwrite);
> +}
> +
> +/*
> + * Does this VMA require the underlying folios to have their dirty state
> + * tracked?
> + */
> +bool vma_needs_dirty_tracking(struct vm_area_struct *vma)
> +{

Sorry for not noticing this earlier, but ...

what about MAP_PRIVATE mappings? When we write, we populate an anon 
page, which will work as expected ... because we don't have to notify 
the fs?

I think you really also want the "If it was private or non-writable, the 
write bit is already clear */" part as well and remove "false" in that case.

Or was there a good reason to disallow private mappings as well?

--
Thanks,

David / dhildenb