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From:   Andreas Dilger <adilger@dilger.ca>
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Subject: Re: [PATCH] ext4: optimize the implementation of ext4_mb_good_group()
Date:   Wed, 12 Aug 2020 16:15:10 -0600
In-Reply-To: <e20b2d8f-1154-adb7-3831-a9e11ba842e9@gmail.com>
Cc:     Theodore Ts'o <tytso@mit.edu>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>
To:     brookxu <brookxu.cn@gmail.com>
References: <e20b2d8f-1154-adb7-3831-a9e11ba842e9@gmail.com>
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On Aug 7, 2020, at 8:01 AM, brookxu <brookxu.cn@gmail.com> wrote:
>=20
> It might be better to adjust the code in two places:
> 1. Determine whether grp is currupt or not should be placed first.
> 2. (cr<=3D2 && free <ac->ac_g_ex.fe_len)should may belong to the crx
>   strategy, and it may be more appropriate to put it in the
>   subsequent switch statement block. For cr1, cr2, the conditions
>   in switch potentially realize the above judgment. For cr0, we
>   should add (free <ac->ac_g_ex.fe_len) judgment, and then delete
>   (free / fragments) >=3D ac->ac_g_ex.fe_len), because cr0 returns
>   true by default.
>=20

> Signed-off-by: Chunguang Xu <brookxu@tencent.com>

This looks correct.  Not quite as simple as moving a few lines around,
but I think the logic is equivalent.

Reviewed-by: Andreas Dilger <adilger@dilger.ca>

> ---
> fs/ext4/mballoc.c | 14 +++++++-------
> 1 file changed, 7 insertions(+), 7 deletions(-)
>=20
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 28a139f..4304113 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -2119,13 +2119,11 @@ static bool ext4_mb_good_group(struct =
ext4_allocation_context *ac,
>=20
> 	BUG_ON(cr < 0 || cr >=3D 4);

This could also potentially be removed and keep only "BUG()" in the
default: case, though it would be good to print the value of "cr".

>=20
> -	free =3D grp->bb_free;
> -	if (free =3D=3D 0)
> -		return false;
> -	if (cr <=3D 2 && free < ac->ac_g_ex.fe_len)
> +	if (unlikely(EXT4_MB_GRP_BBITMAP_CORRUPT(grp)))
> 		return false;
>=20
> -	if (unlikely(EXT4_MB_GRP_BBITMAP_CORRUPT(grp)))
> +	free =3D grp->bb_free;
> +	if (free =3D=3D 0)
> 		return false;
>=20
> 	fragments =3D grp->bb_fragments;
> @@ -2142,8 +2140,10 @@ static bool ext4_mb_good_group(struct =
ext4_allocation_context *ac,
> 		    ((group % flex_size) =3D=3D 0))
> 			return false;
>=20
> -		if ((ac->ac_2order > ac->ac_sb->s_blocksize_bits+1) ||
> -		    (free / fragments) >=3D ac->ac_g_ex.fe_len)
> +		if (free < ac->ac_g_ex.fe_len)
> +			return false;
> +
> +		if (ac->ac_2order > ac->ac_sb->s_blocksize_bits+1)
> 			return true;
>=20
> 		if (grp->bb_largest_free_order < ac->ac_2order)
> --
> 1.8.3.1
>=20


Cheers, Andreas


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