Re: [PATCH v2 3/4] mm: Introduce page_needs_cow_for_dma() for deciding whether cow

Jason Gunthorpe
2021-02-04T23:21:00Z

On Thu, Feb 04, 2021 at 09:50:32AM -0500, Peter Xu wrote:
> We've got quite a few places (pte, pmd, pud) that explicitly checked against
> whether we should break the cow right now during fork(). It's easier to
> provide a helper, especially before we work the same thing on hugetlbfs.
>
> Since we'll reference is_cow_mapping() in mm.h, move it there too. Actually it
> suites mm.h more since internal.h is mm/ only, but mm.h is exported to the
> whole kernel. With that we should expect another patch to use is_cow_mapping()
> whenever we can across the kernel since we do use it quite a lot but it's
> always done with raw code against VM_* flags.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> include/linux/mm.h | 21 +++++++++++++++++++++
> mm/huge_memory.c | 8 ++------
> mm/internal.h | 5 -----
> mm/memory.c | 7 +------
> 4 files changed, 24 insertions(+), 17 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index ecdf8a8cd6ae..6ea20721d349 100644
> +++ b/include/linux/mm.h
> @@ -1291,6 +1291,27 @@ static inline bool page_maybe_dma_pinned(struct page *page)
> GUP_PIN_COUNTING_BIAS;
> }
>
> +static inline bool is_cow_mapping(vm_flags_t flags)
> +{

It feels a bit more logical to pass in a struct vm_area_struct *' here?

Jason