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

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Message-ID: <547753C0.6050604@nod.at>
Date:	Thu, 27 Nov 2014 17:39:28 +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 17:35 schrieb Artem Bityutskiy:
> On Thu, 2014-11-27 at 17:13 +0100, Richard Weinberger wrote:
>> 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.
> 
> Sounds like just you do not need any works and any queues at all. All
> you need is a "please, fastmap me!" flag.
> 
> Then this flag should be checked every time we enter the background
> thread or the fastmap code, and be acted upon.
> 
> So the background thread would first check this flag, and if it is set -
> call the fastmap stuff. The go do the WL works.
> 
> Just off-the top of my head, take with grain of salt.

So you want me to redesign it?
IMHO this is just a matter of taste.

Face it, my brain does not work like yours. I design things differently.

Thanks,
//richard

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