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

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Date:   Mon, 23 May 2022 23:08:06 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ritesh Harjani <ritesh.list@...il.com>
Cc:     Baokun Li <libaokun1@...wei.com>, linux-ext4@...r.kernel.org,
        tytso@....edu, adilger.kernel@...ger.ca, jack@...e.cz,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com,
        yebin10@...wei.com, yukuai3@...wei.com
Subject: Re: [PATCH 2/2] ext4: correct the judgment of BUG in
 ext4_mb_normalize_request

On Tue 24-05-22 01:38:44, Ritesh Harjani wrote:
> On 22/05/21 09:42PM, Baokun Li wrote:
> > When either of the "start + size <= ac->ac_o_ex.fe_logical" or
> > "start > ac->ac_o_ex.fe_logical" conditions is met, it indicates
> > that the fe_logical is not in the allocated range.
> 
> Sounds about right to me based on the logic in ext4_mb_use_inode_pa().
> We try to allocate/preallocate such that ac->ac_o_ex.fe_logical should fall
> within the preallocated range. So if our start or start + size doesn't include
> fe_logical then it is a bug in the ext4_mb_normalize_request() logic.

I agree ac->ac_o_ex.fe_logical is a goal block. But AFAIK it never was a
hard guarantee that we would allocate extent that includes that block. It
was always treated as a hint only. In particular if you look at the logic
in ext4_mb_normalize_request() it does shift the start of the allocation to
avoid preallocated ranges etc. so I don't see how we are guaranteed that
ext4_mb_normalize_request() will result in an allocation request that
includes ac->ac_o_ex.fe_logical.

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR