linux-ext4 - Re: [PATCH] ext4: Don't use CR_BEST_AVAIL

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Message-ID: <ZLWym4n0Z/Ucajbr@debian-BULLSEYE-live-builder-AMD64>
Date:   Mon, 17 Jul 2023 17:28:59 -0400
From:   Eric Whitney <enwlinux@...il.com>
To:     "Ritesh Harjani (IBM)" <ritesh.list@...il.com>
Cc:     linux-ext4@...r.kernel.org, Ojaswin Mujoo <ojaswin@...ux.ibm.com>,
        Eric Whitney <enwlinux@...il.com>
Subject: Re: [PATCH] ext4: Don't use CR_BEST_AVAIL_LEN for non-regular files

* Ritesh Harjani (IBM) <ritesh.list@...il.com>:
> Using CR_BEST_AVAIL_LEN only make sense for regular files, as for
> non-regular files we never normalize the allocation request length i.e.
> goal len is same as original length (ac_g_ex.fe_len == ac_o_ex.fe_len).
> 
> Hence there is no scope of trimming the goal length to make it
> satisfy original request len. Thus this patch avoids using
> CR_BEST_AVAIL_LEN criteria for non-regular files request.
> 
> Fixes: 33122aa930f1 ("ext4: Add allocation criteria 1.5 (CR1_5)")
> Reported-by: Eric Whitney <enwlinux@...il.com>
> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@...il.com>
> ---
>  fs/ext4/mballoc.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 3ab37533349f..bc004f5d3f3c 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -975,7 +975,18 @@ static void ext4_mb_choose_next_group_goal_fast(struct ext4_allocation_context *
>  		*group = grp->bb_group;
>  		ac->ac_flags |= EXT4_MB_CR_GOAL_LEN_FAST_OPTIMIZED;
>  	} else {
> -		*new_cr = CR_BEST_AVAIL_LEN;
> +		/*
> +		 * CR_BEST_AVAIL_LEN works based on the concept that we have
> +		 * a larger normalized goal len request which can be trimmed to
> +		 * a smaller goal len such that it can still satisfy original
> +		 * request len. However, allocation request for non-regular
> +		 * files never gets normalized.
> +		 * See function ext4_mb_normalize_request() (EXT4_MB_HINT_DATA).
> +		 */
> +		if (ac->ac_flags & EXT4_MB_HINT_DATA)
> +			*new_cr = CR_BEST_AVAIL_LEN;
> +		else
> +			*new_cr = CR_GOAL_LEN_SLOW;
>  	}
>  }
> 
> --
> 2.40.1
>

Works for me on 6.5-rc2 with this patch applied - 500/500 generic/269 trials
passed on bigalloc_1k.

Tested-by: Eric Whitney <enwlinux@...il.com>

Thanks!
Eric