linux-kernel - Re: [PATCH v3] xfs: add check before calling xfs_mod

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Date:   Tue, 21 Jun 2022 21:41:09 -0700
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Shida Zhang <starzhangzsd@...il.com>
Cc:     dchinner@...hat.com, zhangshida@...inos.cn,
        linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org
Subject: Re: [PATCH v3] xfs: add check before calling xfs_mod_fdblocks

On Tue, Jun 21, 2022 at 04:42:38PM +0800, Shida Zhang wrote:
> Checks are missing when delta equals 0 in __xfs_ag_resv_free() and
> __xfs_ag_resv_init().
> 
> the case that the delta equals 0 is reachable with the command
> sequence below:
> 
>  # mkfs.xfs -f /dev/sdb5
>  # mount /dev/sdb5 /mnt/scratch/
> 
> where /dev/sdb5 is my disk for test. And if the patch below is
> applied:
> 
> ====
> xfs_mod_freecounter(
>         if (rsvd)
>                 ASSERT(has_resv_pool);
> 
> +       if (delta == 0)
> +               dump_stack();
> +
>         if (delta > 0) {
>                 /*
>                  * If the reserve pool is depleted, put blocks back into it
> ====
> 
> the following stack will be shown in the message:
> 
> =>  xfs_mod_freecounter+0x84/0x2b8
> =>  __xfs_ag_resv_free+0xc4/0x188
> =>  xfs_ag_resv_free+0x24/0x50
> =>  xfs_fs_unreserve_ag_blocks+0x40/0x160
> =>  xfs_mountfs+0x500/0x900
> =>  xfs_fs_fill_super+0x3d8/0x810
> =>  get_tree_bdev+0x164/0x258
> =>  xfs_fs_get_tree+0x20/0x30
> =>  vfs_get_tree+0x30/0xf8
> =>  path_mount+0x3c4/0xa58
> =>  do_mount+0x74/0x98
> 
> =>  xfs_mod_freecounter+0x84/0x2b8
> =>  __xfs_ag_resv_init+0x64/0x1d0
> =>  xfs_ag_resv_init+0x108/0x1c8
> =>  xfs_fs_reserve_ag_blocks+0x4c/0x110
> =>  xfs_mountfs+0x57c/0x900
> =>  xfs_fs_fill_super+0x3d8/0x810
> =>  get_tree_bdev+0x164/0x258
> =>  xfs_fs_get_tree+0x20/0x30
> =>  vfs_get_tree+0x30/0xf8
> =>  path_mount+0x3c4/0xa58
> =>  do_mount+0x74/0x98
> 
> After applying this patch, we can avoid to call xfs_mod_fdblocks when
> delta equals 0.
> 
> Signed-off-by: Shida Zhang <zhangshida@...inos.cn>
> ---
>  Changes from v1:
>  -Add checks before calling xfs_mod_fdblocks instead.
>  Changes from v2:
>  -Rephrase the commit description.
> 
>  fs/xfs/libxfs/xfs_ag_resv.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_ag_resv.c b/fs/xfs/libxfs/xfs_ag_resv.c
> index fe94058d4e9e..c8fa032e4b00 100644
> --- a/fs/xfs/libxfs/xfs_ag_resv.c
> +++ b/fs/xfs/libxfs/xfs_ag_resv.c
> @@ -149,7 +149,12 @@ __xfs_ag_resv_free(
>  		oldresv = resv->ar_orig_reserved;
>  	else
>  		oldresv = resv->ar_reserved;
> -	error = xfs_mod_fdblocks(pag->pag_mount, oldresv, true);
> +
> +	if (oldresv)
> +		error = xfs_mod_fdblocks(pag->pag_mount, oldresv, true);
> +	else
> +		error = 0;
> +
>  	resv->ar_reserved = 0;
>  	resv->ar_asked = 0;
>  	resv->ar_orig_reserved = 0;
> @@ -215,8 +220,13 @@ __xfs_ag_resv_init(
>  
>  	if (XFS_TEST_ERROR(false, mp, XFS_ERRTAG_AG_RESV_FAIL))
>  		error = -ENOSPC;
> -	else
> -		error = xfs_mod_fdblocks(mp, -(int64_t)hidden_space, true);
> +	else {
> +		error = 0;
> +		if (hidden_space)
> +			error = xfs_mod_fdblocks(mp, -(int64_t)hidden_space,
> +						true);

I understand that calling __xfs_ag_resv_init on an AG with a maximally
sized data structure can result in @hidden_space being zero here, but
why does that matter enough to change the code?  Are you experiencing
problems when this happens?  Unnecessary slowdowns at mount time?
Something else?

This is v3 of a patch and I still can't tell why I should care ...?

--D

> +	}
> +
>  	if (error) {
>  		trace_xfs_ag_resv_init_error(pag->pag_mount, pag->pag_agno,
>  				error, _RET_IP_);
> -- 
> 2.25.1
>