linux-kernel - Re: [PATCH 3/4] UBI: Fastmap: Care about the protection queue

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Message-ID: <543C3E60.4080507@nod.at>
Date:	Mon, 13 Oct 2014 23:04:32 +0200
From:	Richard Weinberger <richard@....at>
To:	dedekind1@...il.com
CC:	linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] UBI: Fastmap: Care about the protection queue

Am 13.10.2014 um 17:23 schrieb Artem Bityutskiy:
> Well, used and free are RB-trees, looking them up is slow.

This is true but we'd have to look it up in multiple trees and the protection queue...

> If what you need is to go through all used and free PEBs, then you can
> introduce some kind of
> 
> struct ubi_wl_entry *ubi_wl_get_next_used(struct ubi_wl_entry *prev)
> 
> function, and similar functions for the free.
> 
> I would return you the next entry (or NULL would indicate the end), and
> it would take the previous entry as the input, or NULL for the first
> call.
> 
> We'd need to take the locks before calling this function. This is
> cleaner than what we do now, right?

ubi_update_fastmap() takes ubi->wl_lock anyway to block any changes in the free, used, etc. trees
to make sure that the to be taken state snapshot is consistent.

>> But we could add the state information to struct ubi_wl_entry by adding a single integer attribute called "state" or "flags".
> 
> But there is a price - memory consumption. We do not want to pay it just
> for making the inter-subsystems boundaries better, there ought to be a
> better reason.
> 
> Say, for an (imaginary) 8GiB NAND chip with 128KiB PEB size this would
> cost 256KiB of RAM.

Is 128KiB PEB size still realistic on modern NANDs?
Even if, 256KiB are not much and the kernel consumes this additionally with
every new release.
But I can understand your concerns.

> Squeezing the state into the last 2 bits a memory reference would be
> another possibility, BTW. Not elegant, though...
> 
>> Would this make you happy? :)
> 
> Not very, I'd save this for the last resort solution.

Okay, I'll try harder to make you happy.

Thanks,
//richard
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