linux-kernel - Re: [PATCH v3 2/3] ext4: correct the judgment of BUG in ext4_mb_normalize

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Message-ID: <20220528151219.ojgwg7pct6ci5zd6@riteshh-domain>
Date:   Sat, 28 May 2022 20:42:19 +0530
From:   Ritesh Harjani <ritesh.list@...il.com>
To:     Baokun Li <libaokun1@...wei.com>
Cc:     linux-ext4@...r.kernel.org, tytso@....edu,
        adilger.kernel@...ger.ca, jack@...e.cz, lczerner@...hat.com,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com,
        yebin10@...wei.com, yukuai3@...wei.com
Subject: Re: [PATCH v3 2/3] ext4: correct the judgment of BUG in
 ext4_mb_normalize_request

On 22/05/28 07:00PM, Baokun Li wrote:
> ext4_mb_normalize_request() can move logical start of allocated blocks
> to reduce fragmentation and better utilize preallocation. However logical
> block requested as a start of allocation (ac->ac_o_ex.fe_logical) should
> always be covered by allocated blocks so we should check that by
> modifying and to or in the assertion.
>
> Signed-off-by: Baokun Li <libaokun1@...wei.com>

Changes looks good to me as we discussed. Feel free to add -

Reviewed-by: Ritesh Harjani <ritesh.list@...il.com>

> ---
> V1->V2:
> 	Change Fixes from dfe076c106f6 to c9de560ded61.
> V2->V3:
> 	Delete Fixes tag.
> 	Add more comments and commit logs to make the code easier to understand.
>
>  fs/ext4/mballoc.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
>
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 4d3740fdff90..9e06334771a3 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -4185,7 +4185,22 @@ ext4_mb_normalize_request(struct ext4_allocation_context *ac,
>  	}
>  	rcu_read_unlock();
>
> -	if (start + size <= ac->ac_o_ex.fe_logical &&
> +	/*
> +	 * In this function "start" and "size" are normalized for better
> +	 * alignment and length such that we could preallocate more blocks.
> +	 * This normalization is done such that original request of
> +	 * ac->ac_o_ex.fe_logical & fe_len should always lie within "start" and
> +	 * "size" boundaries.
> +	 * (Note fe_len can be relaxed since FS block allocation API does not
> +	 * provide gurantee on number of contiguous blocks allocation since that
> +	 * depends upon free space left, etc).
> +	 * In case of inode pa, later we use the allocated blocks
> +	 * [pa_start + fe_logical - pa_lstart, fe_len/size] from the preallocated
> +	 * range of goal/best blocks [start, size] to put it at the
> +	 * ac_o_ex.fe_logical extent of this inode.
> +	 * (See ext4_mb_use_inode_pa() for more details)
> +	 */
> +	if (start + size <= ac->ac_o_ex.fe_logical ||
>  			start > ac->ac_o_ex.fe_logical) {
>  		ext4_msg(ac->ac_sb, KERN_ERR,
>  			 "start %lu, size %lu, fe_logical %lu",
> --
> 2.31.1
>