linux-kernel - Re: [PATCH v2 3/4] mm: Introduce page_needs_cow_for

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Date:   Thu, 4 Feb 2021 19:20:00 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Kirill Shutemov <kirill@...temov.name>,
        Wei Zhang <wzam@...zon.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Matthew Wilcox <willy@...radead.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Gal Pressman <galpress@...zon.com>, Jan Kara <jack@...e.cz>,
        Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        David Gibson <david@...son.dropbear.id.au>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2 3/4] mm: Introduce page_needs_cow_for_dma() for
 deciding whether cow

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@...hat.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