linux-kernel - Different writeback timing since v5.14

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Date:   Tue, 1 Mar 2022 11:11:57 +0100 (CET)
From:   Richard Weinberger <richard@....at>
To:     wuchi.zero@...il.com
Cc:     Rafał Miłecki <zajec5@...il.com>,
        jack@...e.cz, tj@...nel.org, mszeredi@...hat.com,
        sedat.dilek@...il.com, axboe@...com, akpm@...ux-foundation.org,
        torvalds@...ux-foundation.org, linux-mm <linux-mm@...ck.org>,
        linux-mtd <linux-mtd@...ts.infradead.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Different writeback timing since v5.14

Hi!

Rafał and I discovered that page writeback on UBIFS behaves different since v5.14.
When a simple write, such as "echo foo > /mnt/ubibfs/bar.txt", happens it takes
a few seconds until writeback calls ubifs_writepage().

Before commit ab19939a6a50 ("mm/page-writeback: Fix performance when BDI's share of ratio is 0.")
it was 30 seconds (vm.dirty_expire_centisecs), after this change it happens after 5 seconds
(vm.dirty_writeback_centisecs).

Is this expected?
Just want to make sure that the said commit didn't uncover an UBIFS issue.

Thanks,
//richard