linux-kernel - Re: [RESEND PATCH v2] xfs: fix the entry condition of exact EOF block allocation optimization

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Message-ID: <Z09stGvgxKV91XfX@dread.disaster.area>
Date: Wed, 4 Dec 2024 07:40:20 +1100
From: Dave Chinner <david@...morbit.com>
To: Jinliang Zheng <alexjlzheng@...il.com>
Cc: cem@...nel.org, djwong@...nel.org, hch@...radead.org,
	dchinner@...hat.com, chandanbabu@...nel.org,
	linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jinliang Zheng <alexjlzheng@...cent.com>
Subject: Re: [RESEND PATCH v2] xfs: fix the entry condition of exact EOF
 block allocation optimization

On Sat, Nov 30, 2024 at 07:11:32PM +0800, Jinliang Zheng wrote:
> When we call create(), lseek() and write() sequentially, offset != 0
> cannot be used as a judgment condition for whether the file already
> has extents.
> 
> Furthermore, when xfs_bmap_adjacent() has not given a better blkno,
> it is not necessary to use exact EOF block allocation.
> 
> Signed-off-by: Jinliang Zheng <alexjlzheng@...cent.com>
> ---
> Changelog:
> - V2: Fix the entry condition
> - V1: https://lore.kernel.org/linux-xfs/ZyFJm7xg7Msd6eVr@dread.disaster.area/T/#t
> ---
>  fs/xfs/libxfs/xfs_bmap.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index 36dd08d13293..c1e5372b6b2e 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -3531,12 +3531,14 @@ xfs_bmap_btalloc_at_eof(
>  	int			error;
>  
>  	/*
> -	 * If there are already extents in the file, try an exact EOF block
> -	 * allocation to extend the file as a contiguous extent. If that fails,
> -	 * or it's the first allocation in a file, just try for a stripe aligned
> -	 * allocation.
> +	 * If there are already extents in the file, and xfs_bmap_adjacent() has
> +	 * given a better blkno, try an exact EOF block allocation to extend the
> +	 * file as a contiguous extent. If that fails, or it's the first
> +	 * allocation in a file, just try for a stripe aligned allocation.
>  	 */
> -	if (ap->offset) {
> +	if (ap->prev.br_startoff != NULLFILEOFF &&
> +	     !isnullstartblock(ap->prev.br_startblock) &&
> +	     xfs_bmap_adjacent_valid(ap, ap->blkno, ap->prev.br_startblock)) {

There's no need for calling xfs_bmap_adjacent_valid() here -
we know that ap->blkno is valid because the
bounds checking has already been done by xfs_bmap_adjacent().

Actually, for another patch, the bounds checking in
xfs_bmap_adjacent_valid() is incorrect. What happens if the last AG
is a runt? i.e. it open codes xfs_verify_fsbno() and gets it wrong.

-Dave.
-- 
Dave Chinner
david@...morbit.com