linux-kernel - Re: [PATCH 2/6] UBI: Fastmap: Ensure that only one fastmap work is scheduled

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Message-ID: <54774DB7.5090200@nod.at>
Date:	Thu, 27 Nov 2014 17:13:43 +0100
From:	Richard Weinberger <richard@....at>
To:	dedekind1@...il.com
CC:	linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/6] UBI: Fastmap: Ensure that only one fastmap work is
 scheduled

Am 27.11.2014 um 16:27 schrieb Artem Bityutskiy:
> On Mon, 2014-11-24 at 14:20 +0100, Richard Weinberger wrote:
>> If the WL pool runs out of PEBs we schedule a fastmap write
>> to refill it as soon as possible.
>> Ensure that only one at a time is scheduled otherwise we might end in
>> a fastmap write storm because writing the fastmap can schedule another
>> write if bitflips are detected.
> 
> Could you please provide some more details about the "write storm". Does
> it happen when there are 2 fastmap works in the queue? Or if they run
> simultaneously? Why the storm happens and white kind of "writes" it
> consists of?

If UBI needs to write a new fastmap while wear leveling (by using get_peb_for_wl())
a fastmap work is scheduled.
We cannot write a fastmap in this context because we're in atomic context.
At this point one fastmap write is scheduled. If now get_peb_for_wl() is executed
a second time it will schedule another fastmap work because the pools are still not refilled.
This will go on until finally a fastmap got written. Either by the the kworker or
ubi_wl_get_peb().
As now a lot of fastmap works are scheduled they all will be issues in series.
Hence, a write storm. :)

Thanks,
//richard
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