linux-kernel - Re: [PATCH 1/3] mm: add vm_insert_mixed

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Date:   Fri, 9 Jun 2017 14:23:51 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-xfs@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Jonathan Corbet <corbet@....net>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-doc@...r.kernel.org, linux-mm <linux-mm@...ck.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Ingo Molnar <mingo@...hat.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>,
        ext4 hackers <linux-ext4@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>
Subject: Re: [PATCH 1/3] mm: add vm_insert_mixed_mkwrite()

On Wed, Jun 7, 2017 at 1:48 PM, Ross Zwisler
<ross.zwisler@...ux.intel.com> wrote:
> To be able to use the common 4k zero page in DAX we need to have our PTE
> fault path look more like our PMD fault path where a PTE entry can be
> marked as dirty and writeable as it is first inserted, rather than waiting
> for a follow-up dax_pfn_mkwrite() => finish_mkwrite_fault() call.
>
> Right now we can rely on having a dax_pfn_mkwrite() call because we can
> distinguish between these two cases in do_wp_page():
>
>         case 1: 4k zero page => writable DAX storage
>         case 2: read-only DAX storage => writeable DAX storage
>
> This distinction is made by via vm_normal_page().  vm_normal_page() returns
> false for the common 4k zero page, though, just as it does for DAX ptes.
> Instead of special casing the DAX + 4k zero page case, we will simplify our
> DAX PTE page fault sequence so that it matches our DAX PMD sequence, and
> get rid of dax_pfn_mkwrite() completely.
>
> This means that insert_pfn() needs to follow the lead of insert_pfn_pmd()
> and allow us to pass in a 'mkwrite' flag.  If 'mkwrite' is set insert_pfn()
> will do the work that was previously done by wp_page_reuse() as part of the
> dax_pfn_mkwrite() call path.
>
> Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
> ---
>  include/linux/mm.h |  9 +++++++--
>  mm/memory.c        | 21 ++++++++++++++-------
>  2 files changed, 21 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index b892e95..11e323a 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2294,10 +2294,15 @@ int vm_insert_pfn(struct vm_area_struct *vma, unsigned long addr,
>                         unsigned long pfn);
>  int vm_insert_pfn_prot(struct vm_area_struct *vma, unsigned long addr,
>                         unsigned long pfn, pgprot_t pgprot);
> -int vm_insert_mixed(struct vm_area_struct *vma, unsigned long addr,
> -                       pfn_t pfn);
> +int vm_insert_mixed_mkwrite(struct vm_area_struct *vma, unsigned long addr,
> +                       pfn_t pfn, bool mkwrite);

Are there any other planned public users of vm_insert_mixed_mkwrite()
that would pass false? I think not.

>  int vm_iomap_memory(struct vm_area_struct *vma, phys_addr_t start, unsigned long len);
>
> +static inline int vm_insert_mixed(struct vm_area_struct *vma,
> +               unsigned long addr, pfn_t pfn)
> +{
> +       return vm_insert_mixed_mkwrite(vma, addr, pfn, false);
> +}

...in other words instead of making the distinction of
vm_insert_mixed_mkwrite() and vm_insert_mixed() with extra flag
argument just move the distinction into mm/memory.c directly.

So, the prototype remains the same as vm_insert_mixed()

int vm_insert_mixed_mkwrite(struct vm_area_struct *vma, unsigned long
addr, pfn_t pfn);

...and only static insert_pfn(...) needs to change.