linux-kernel - Re: [PATCH 03/21] ext4: avoid to use preallocated blocks if EXT4_MB_HINT_GOAL

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Date:   Mon, 13 Feb 2023 14:08:19 +0530
From:   Ojaswin Mujoo <ojaswin@...ux.ibm.com>
To:     Kemeng Shi <shikemeng@...weicloud.com>
Cc:     tytso@....edu, adilger.kernel@...ger.ca, jack@...e.cz,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/21] ext4: avoid to use preallocated blocks if
 EXT4_MB_HINT_GOAL_ONLY is set

On Fri, Feb 10, 2023 at 03:48:07AM +0800, Kemeng Shi wrote:
> ext4_mb_use_preallocated will ignore the demand to alloc at goal block
> only. Return false if EXT4_MB_HINT_GOAL_ONLY is set before use
> preallocated blocks in ext4_mb_use_preallocated.
> 
> Signed-off-by: Kemeng Shi <shikemeng@...weicloud.com>
> ---
>  fs/ext4/mballoc.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 375d9655b525..352ac9139fee 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -4368,6 +4368,9 @@ ext4_mb_use_preallocated(struct ext4_allocation_context *ac)
>  	if (!(ac->ac_flags & EXT4_MB_HINT_DATA))
>  		return false;
>  
> +	if (unlikely(ac->ac_flags & EXT4_MB_HINT_GOAL_ONLY))
> +		return false;
> +
>  	/* first, try per-file preallocation */
>  	rcu_read_lock();
>  	list_for_each_entry_rcu(pa, &ei->i_prealloc_list, pa_inode_list) {
> -- 
> 2.30.0
> 
So with the flag, even when we request for a logical/goal block
combination that can be satisfied by one of the inode PAs in the list,
we would still exit early and the allocation would eventually fail since
the goal block would be marked "in-use" in the buddy. This doesn't seem
to be desirable.

Maybe instead of exiting early we should try to find a PA that satisfies
the logical block we are asking for and then incase of
EXT4_MB_HINT_GOAL_ONLY, we can add a check to see if the physical block
of the PA corresponds to the goal block. Would like to hear your
thoughts on this.

Regards,
ojaswin