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Date:   Tue, 1 Mar 2022 16:05:54 +0100 (CET)
From:   Richard Weinberger <richard@nod.at>
To:     Jan Kara <jack@suse.cz>
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Subject: Re: Different writeback timing since v5.14
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Thread-Topic: Different writeback timing since v5.14
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Jan,

----- Ursprüngliche Mail -----
> Von: "Jan Kara" <jack@suse.cz>
>> Is this expected?
>> Just want to make sure that the said commit didn't uncover an UBIFS issue.
> 
> Yes, I think it is expected. Likely the background threshold for UBIFS bdi
> is very small (probably UBIFS is not used much for writeback compared to
> other filesystems). Previously, we just used wb_stat() which returned 0
> (PCP counter inexact value) and so background writeback didn't trigger. Now
> we use wb_stat_sum() when threshold is small, get exact value of dirty
> pages and decide to start background writeback.

Thanks for the prompt reply!

> The only thing is, whether it is really expected that the threshold for
> UBIFS bdi is so small. You can check the values in
> /sys/kernel/debug/bdi/<bdi>/stats.

BdiDirtyThresh is indeed 0.

BdiWriteback:                0 kB
BdiReclaimable:              0 kB
BdiDirtyThresh:              0 kB
DirtyThresh:            772620 kB
BackgroundThresh:       385836 kB
BdiDirtied:                  0 kB
BdiWritten:                  0 kB
BdiWriteBandwidth:      102400 kBps
b_dirty:                     0
b_io:                        0
b_more_io:                   0
b_dirty_time:                0
bdi_list:                    1
state:                       1

Thanks,
//richard