linux-kernel - Re: [PATCH v7 3/8] fsdax: Replace mmap entry in case of CoW

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Date:   Thu, 19 Aug 2021 15:54:01 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Shiyang Ruan <ruansy.fnst@...itsu.com>
Cc:     "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Hellwig <hch@....de>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        david <david@...morbit.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux NVDIMM <nvdimm@...ts.linux.dev>,
        Goldwyn Rodrigues <rgoldwyn@...e.de>,
        Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>,
        Goldwyn Rodrigues <rgoldwyn@...e.com>,
        Ritesh Harjani <riteshh@...ux.ibm.com>
Subject: Re: [PATCH v7 3/8] fsdax: Replace mmap entry in case of CoW

On Sun, Aug 15, 2021 at 11:05 PM Shiyang Ruan <ruansy.fnst@...itsu.com> wrote:
>
> We replace the existing entry to the newly allocated one in case of CoW.
> Also, we mark the entry as PAGECACHE_TAG_TOWRITE so writeback marks this
> entry as writeprotected.  This helps us snapshots so new write
> pagefaults after snapshots trigger a CoW.
>
> Signed-off-by: Goldwyn Rodrigues <rgoldwyn@...e.com>
> Signed-off-by: Shiyang Ruan <ruansy.fnst@...itsu.com>
> Reviewed-by: Christoph Hellwig <hch@....de>
> Reviewed-by: Ritesh Harjani <riteshh@...ux.ibm.com>
> Reviewed-by: Darrick J. Wong <djwong@...nel.org>
> ---
>  fs/dax.c | 39 ++++++++++++++++++++++++++++-----------
>  1 file changed, 28 insertions(+), 11 deletions(-)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index 697a7b7bb96f..e49ba68cc7e4 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -734,6 +734,10 @@ static int copy_cow_page_dax(struct block_device *bdev, struct dax_device *dax_d
>         return 0;
>  }
>
> +/* DAX Insert Flag: The state of the entry we insert */
> +#define DAX_IF_DIRTY           (1 << 0)
> +#define DAX_IF_COW             (1 << 1)
> +
>  /*
>   * By this point grab_mapping_entry() has ensured that we have a locked entry
>   * of the appropriate size so we don't have to worry about downgrading PMDs to
> @@ -741,16 +745,19 @@ static int copy_cow_page_dax(struct block_device *bdev, struct dax_device *dax_d
>   * already in the tree, we will skip the insertion and just dirty the PMD as
>   * appropriate.
>   */
> -static void *dax_insert_entry(struct xa_state *xas,
> -               struct address_space *mapping, struct vm_fault *vmf,
> -               void *entry, pfn_t pfn, unsigned long flags, bool dirty)
> +static void *dax_insert_entry(struct xa_state *xas, struct vm_fault *vmf,
> +               void *entry, pfn_t pfn, unsigned long flags,
> +               unsigned int insert_flags)

I'm late, so feel free to ignore this style feedback, but what about
changing the signature to:

static void *dax_insert_entry(struct xa_state *xas, struct vm_fault *vmf,
                              const struct iomap_iter *iter, void
*entry, pfn_t pfn,
                              unsigned long flags)


>  {
> +       struct address_space *mapping = vmf->vma->vm_file->f_mapping;
>         void *new_entry = dax_make_entry(pfn, flags);
> +       bool dirty = insert_flags & DAX_IF_DIRTY;
> +       bool cow = insert_flags & DAX_IF_COW;

...and then calculate these flags from the source data. I'm just
reacting to "yet more flags".

So, take it or leave it,

Reviewed-by: Dan Williams <dan.j.williams@...el.com>