linux-ext4 - [Bug 217965] ext4(?) regression since 6.5.0 on sata hdd

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Message-ID: <bug-217965-13602-vGzi4pJCMX@https.bugzilla.kernel.org/>
Date: Wed, 27 Aug 2025 17:07:17 +0000
From: bugzilla-daemon@...nel.org
To: linux-ext4@...r.kernel.org
Subject: [Bug 217965] ext4(?) regression since 6.5.0 on sata hdd

https://bugzilla.kernel.org/show_bug.cgi?id=217965

--- Comment #73 from Ojaswin Mujoo (ojaswin.mujoo@....com) ---
Hi Mingyu,

Thanks for looking into this and sharing the reproducer. You are correct in the
analysis that we were getting stuck trying to check the same block group for
aligned blocks, due to the fact that the fragment lists always returned the
same group. 

However, about this:

> The author changed the fragment order RB tree into list for better
> performance.
> However, the function 'ext4_mb_find_good_group_avg_frag_lists' will always 
> returns the same group every time,

The change from rbtree to list was not done in the patchset you listed but
rather here:

https://lore.kernel.org/all/20220908092136.11770-5-jack@suse.cz/

which got merged in v6.0 kernel hence I think the behavior of fragment lists
returning the same block group was there even before my patchset (the one you
listed) However I remember people had mentioned that they started to see it
after v6.5. 

Its been sometime since I looked into this but I remember I had concluded that
since my patchset added new allocation criteria which made the allocator trim
the request to more aggressively look for BGs in the free fragment lists, we
just made this bug more easier to hit.

I tried the replicator however I was unable to get to the high CPU util, but
since you already have the setup, can you check if you are able to hit this
issue in v6.4 vs v6.5.

Thanks again,
Ojaswin

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