linux-ext4 - Re: [PATCH] ext4: correct best extent lstart adjustment logic

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Date: Thu, 1 Feb 2024 12:46:14 +0100
From: Jan Kara <jack@...e.cz>
To: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
Cc: Baokun Li <libaokun1@...wei.com>, Jan Kara <jack@...e.cz>,
	linux-ext4@...r.kernel.org, tytso@....edu, adilger.kernel@...ger.ca,
	ritesh.list@...il.com, linux-kernel@...r.kernel.org,
	yi.zhang@...wei.com, yangerkun@...wei.com, yukuai3@...wei.com,
	stable@...nel.org
Subject: Re: [PATCH] ext4: correct best extent lstart adjustment logic

Hi guys!

On Thu 01-02-24 16:38:33, Ojaswin Mujoo wrote:
> Thanks for the CC, I somehow missed this patch.
> 
> As described in the discussion Jan linked [1] , there is a known bug in the
> normalize code (which i should probably get back to now ) where we sometimes
> end up with a goal range which doesn't completely cover the original extent and
> this was causing issues when we tried to cover the complete original request in
> the PA window adjustment logic. That and to minimize fragmentation, we ended up
> going with the logic we have right now.
> 
> In short, I agree that in the example Baokun pointed out, it is not
> optimal to have to make an allocation request twice when we can get it in
> one go.
> 
> I also think Baokun is correct that if keeping the best extent at the end
> doesn't cover the original start, then any other case should not lead to
> it overflowing out of goal extent, including the case where original
> extent is overflowing goal extent.

Right, it was not obvious to me yesterday but when I've now reread how the
normalization shifts the goal window, it is obvious.

> So, as mentioned, it boils down to a trade off between multiple allocations and slightly 
> increased fragmentation. iiuc preallocations are anyways dropped when the file closes
> so I think it shouldn't hurt too much fragmentation wise to prioritize less
> allocations. What are your thoughts on this Jan, Baokun?

OK, I'm fine with the Baokun's change if we remove the problematic BUG_ON.

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