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Re: [PATCH 4/8] xfs: add DAX block zeroing support

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 4/8] xfs: add DAX block zeroing support
From: Brian Foster <bfoster@xxxxxxxxxx>
Date: Mon, 6 Apr 2015 13:48:53 -0400
Cc: xfs@xxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, willy@xxxxxxxxxxxxxxx, jack@xxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1427194266-2885-5-git-send-email-david@xxxxxxxxxxxxx>
References: <1427194266-2885-1-git-send-email-david@xxxxxxxxxxxxx> <1427194266-2885-5-git-send-email-david@xxxxxxxxxxxxx>
User-agent: Mutt/1.5.23 (2014-03-12)
On Tue, Mar 24, 2015 at 09:51:02PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
> 
> Add initial support for DAX block zeroing operations to XFS. DAX
> cannot use buffered IO through the page cache for zeroing, nor do we
> need to issue IO for uncached block zeroing. In both cases, we can
> simply call out to the dax block zeroing function.
> 
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> ---
>  fs/xfs/xfs_bmap_util.c | 23 +++++++++++++++++++----
>  fs/xfs/xfs_file.c      | 28 +++++++++++++++++-----------
>  2 files changed, 36 insertions(+), 15 deletions(-)
> 
> diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
> index 1bd5393..d1fe432 100644
> --- a/fs/xfs/xfs_bmap_util.c
> +++ b/fs/xfs/xfs_bmap_util.c
> @@ -1133,14 +1133,29 @@ xfs_zero_remaining_bytes(
>                       break;
>               ASSERT(imap.br_blockcount >= 1);
>               ASSERT(imap.br_startoff == offset_fsb);
> +             ASSERT(imap.br_startblock != DELAYSTARTBLOCK);
> +
> +             if (imap.br_startblock == HOLESTARTBLOCK ||
> +                 imap.br_state == XFS_EXT_UNWRITTEN) {
> +                     /* skip the entire extent */
> +                     lastoffset = XFS_FSB_TO_B(mp, imap.br_startoff +
> +                                                   imap.br_blockcount) - 1;
> +                     continue;
> +             }
> +
>               lastoffset = XFS_FSB_TO_B(mp, imap.br_startoff + 1) - 1;
>               if (lastoffset > endoff)
>                       lastoffset = endoff;
> -             if (imap.br_startblock == HOLESTARTBLOCK)
> -                     continue;
> -             ASSERT(imap.br_startblock != DELAYSTARTBLOCK);
> -             if (imap.br_state == XFS_EXT_UNWRITTEN)
> +
> +             /* DAX can just zero the backing device directly */
> +             if (IS_DAX(VFS_I(ip))) {
> +                     error = dax_zero_page_range(VFS_I(ip), offset,
> +                                                 lastoffset - offset + 1,
> +                                                 xfs_get_blocks_dax);
> +                     if (error)
> +                             return error;
>                       continue;
> +             }
>  
>               error = xfs_buf_read_uncached(XFS_IS_REALTIME_INODE(ip) ?
>                               mp->m_rtdev_targp : mp->m_ddev_targp,
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index a4c882e..94713c2 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -97,7 +97,8 @@ xfs_iozero(
>  {
>       struct page             *page;
>       struct address_space    *mapping;
> -     int                     status;
> +     int                     status = 0;
> +
>  
>       mapping = VFS_I(ip)->i_mapping;
>       do {
> @@ -109,20 +110,25 @@ xfs_iozero(
>               if (bytes > count)
>                       bytes = count;
>  
> -             status = pagecache_write_begin(NULL, mapping, pos, bytes,
> -                                     AOP_FLAG_UNINTERRUPTIBLE,
> -                                     &page, &fsdata);
> -             if (status)
> -                     break;
> +             if (IS_DAX(VFS_I(ip)))
> +                     dax_zero_page_range(VFS_I(ip), pos, bytes,
> +                                                xfs_get_blocks_dax);

xfs_get_blocks_dax() isn't defined yet. We should also probably error
check here, yes?

A nit... If we have to update this patch, it would be nice to update the
comment above the function to adjust expectations with regard to the
suggestion that this always allocates blocks. If I follow the dax
codepath correctly, dax_zero_page_range() is a noop over holes or
unwritten blocks (not that it seems to matter for current callers).

> +             else {
> +                     status = pagecache_write_begin(NULL, mapping, pos, 
> bytes,
> +                                             AOP_FLAG_UNINTERRUPTIBLE,
> +                                             &page, &fsdata);
> +                     if (status)
> +                             break;
>  
> -             zero_user(page, offset, bytes);
> +                     zero_user(page, offset, bytes);
>  
> -             status = pagecache_write_end(NULL, mapping, pos, bytes, bytes,
> -                                     page, fsdata);
> -             WARN_ON(status <= 0); /* can't return less than zero! */
> +                     status = pagecache_write_end(NULL, mapping, pos, bytes,
> +                                             bytes, page, fsdata);
> +                     WARN_ON(status <= 0); /* can't return less than zero! */
> +                     status = 0;
> +             }
>               pos += bytes;
>               count -= bytes;
> -             status = 0;
>       } while (count);
>  
>       return (-status);

FWIW, that looks like a potential positive return code path
(write_begin)...

Brian

> -- 
> 2.0.0
> 
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