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Re: [PATCH v2 2/7] xfs: push rounddown_pow_of_two() to after prealloc th

To: Brian Foster <bfoster@xxxxxxxxxx>
Subject: Re: [PATCH v2 2/7] xfs: push rounddown_pow_of_two() to after prealloc throttle
From: Ben Myers <bpm@xxxxxxx>
Date: Tue, 22 Jan 2013 16:47:16 -0600
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1357150091-37530-3-git-send-email-bfoster@xxxxxxxxxx>
References: <1357150091-37530-1-git-send-email-bfoster@xxxxxxxxxx> <1357150091-37530-3-git-send-email-bfoster@xxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
Hey Brian,

On Wed, Jan 02, 2013 at 01:08:06PM -0500, Brian Foster wrote:
> The round down occurs towards the beginning of the function. Push
> it down after throttling has occurred. This is to support adding > further 
> transformations to 'alloc_blocks' that might not preserve
> power-of-two alignment (and thus could lead to rounding down
> multiple times).
> 
> Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx>
> ---
>  fs/xfs/xfs_iomap.c |   21 +++++++++++++++------
>  1 files changed, 15 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index bd7c060..b2002a5 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -329,13 +329,14 @@ xfs_iomap_prealloc_size(
>               goto check_writeio;
>  
>       /*
> -      * rounddown_pow_of_two() returns an undefined result
> -      * if we pass in alloc_blocks = 0. Hence the "+ 1" to
> -      * ensure we always pass in a non-zero value.
> +      * MAXEXTLEN is not a power of two value but we round the prealloc down
> +      * to the nearest power of two value after throttling. To prevent the
> +      * round down from unconditionally reducing the maximum supported 
> prealloc
> +      * size, we round up first, apply appropriate throttling, round down and
> +      * cap the value to MAXEXTLEN.
>        */
> -     alloc_blocks = XFS_B_TO_FSB(mp, XFS_ISIZE(ip)) + 1;
> -     alloc_blocks = XFS_FILEOFF_MIN(MAXEXTLEN,
> -                             rounddown_pow_of_two(alloc_blocks));
> +     alloc_blocks = XFS_FILEOFF_MIN(roundup_pow_of_two(MAXEXTLEN),
> +                             XFS_B_TO_FSB(mp, XFS_ISIZE(ip)));
>  
>       xfs_icsb_sync_counters(mp, XFS_ICSB_LAZY_COUNT);
>       freesp = mp->m_sb.sb_fdblocks;
> @@ -352,6 +353,14 @@ xfs_iomap_prealloc_size(
>       }
>       if (shift)
>               alloc_blocks >>= shift;
> +     /*
> +      * rounddown_pow_of_two() returns an undefined result if we pass in
> +      * alloc_blocks = 0.
> +      */
> +     if (alloc_blocks)
> +             alloc_blocks = rounddown_pow_of_two(alloc_blocks);
> +     if (alloc_blocks > MAXEXTLEN)
> +             alloc_blocks = MAXEXTLEN;

Well that hurt my brain cell.  It turns out:

xfs_fsblock_t   len, len2, len3;
len = MAXEXTLEN;
len2 = roundup_pow_of_two(len);
len3 = rounddown_pow_of_two(len2);

printk("maxextlen %lu up one %lu down one %lu\n", len, len2, len3);

prints:
maxextlen 2097151 up one 2097152 down one 2097152

Looks good Brian.

Reviewed-by: Ben Myers <bpm@xxxxxxx>

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