linux-ext4 - Re: [PATCH v2] ext4: delete invalid ac_b_extent backup inside ext4_mb_use_best

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Date:   Thu, 13 Aug 2020 02:44:24 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     brookxu <brookxu.cn@...il.com>
Cc:     Theodore Ts'o <tytso@....edu>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH v2] ext4: delete invalid ac_b_extent backup inside
 ext4_mb_use_best_found()

On Aug 7, 2020, at 5:32 AM, brookxu <brookxu.cn@...il.com> wrote:
> 
> Delete invalid ac_b_extent backup inside ext4_mb_use_best_found(),
> we have done this operation in ext4_mb_new_group_pa() and
> ext4_mb_new_inode_pa().

I'm not sure I understand this patch completely.

The calls to ext4_mb_new_group_pa() and ext4_mb_new_inode_pa() are
done from ext4_mb_new_preallocation(), which is called at the *end*
of ext4_mb_use_best_found() (i.e. after the lines that are being
deleted).

Maybe I'm confused by the description "we *have done* this operation"
makes it seem like it was already done, but really it should be
"we *will do* this operation in ..."?

That said, it would make more sense to keep the one line here in
ext4_mb_use_best_found() and remove the two duplicate lines in
ext4_mb_new_group_pa() and ext4_mb_new_inode_pa()?  In that case,
the patch description would be more correct, like:

    Delete duplicate ac_b_extent backup in ext4_mb_new_group_pa()
    and ext4_mb_new_inode_pa(), since we have done this operation
    in ext4_mb_use_best_found() already.

Cheers, Andreas

PS: thank you for taking the time to look at this code and improve it.
I know it is complex and hard to understand, but going through it like
this and trimming off the bad bits makes it a bit easier to understand
and maintain with each small patch.

> Signed-off-by: Chunguang Xu <brookxu@...cent.com>
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 9b1c3ad..fb63e9f 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -1704,10 +1704,6 @@ static void ext4_mb_use_best_found(struct ext4_allocation_context *ac,
> 	ac->ac_b_ex.fe_logical = ac->ac_g_ex.fe_logical;
> 	ret = mb_mark_used(e4b, &ac->ac_b_ex);
> 
> -	/* preallocation can change ac_b_ex, thus we store actually
> -	 * allocated blocks for history */
> -	ac->ac_f_ex = ac->ac_b_ex;
> -
> 	ac->ac_status = AC_STATUS_FOUND;
> 	ac->ac_tail = ret & 0xffff;
> 	ac->ac_buddy = ret >> 16;
> @@ -1726,8 +1722,8 @@ static void ext4_mb_use_best_found(struct ext4_allocation_context *ac,
> 	/* store last allocated for subsequent stream allocation */
> 	if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
> 		spin_lock(&sbi->s_md_lock);
> -		sbi->s_mb_last_group = ac->ac_f_ex.fe_group;
> -		sbi->s_mb_last_start = ac->ac_f_ex.fe_start;
> +		sbi->s_mb_last_group = ac->ac_b_ex.fe_group;
> +		sbi->s_mb_last_start = ac->ac_b_ex.fe_start;
> 		spin_unlock(&sbi->s_md_lock);
> 	}
> 	/*
> --
> 1.8.3.1

Cheers, Andreas

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