linux-kernel - Re: Raid 0 Swap?

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Date:	Mon, 18 Sep 2006 11:50:22 +0200
From:	Denis Vlasenko <vda.linux@...glemail.com>
To:	Jan Engelhardt <jengelh@...ux01.gwdg.de>
Cc:	Michael Tokarev <mjt@....msk.ru>,
	Helge Hafting <helge.hafting@...el.hist.no>,
	Marc Perkel <marc@...kel.com>, linux-kernel@...r.kernel.org
Subject: Re: Raid 0 Swap?

On Monday 04 September 2006 12:46, Jan Engelhardt wrote:
> >> I thought kernel data weren't swapped at all?
> 
> If the swap code was swapped, who would swap it in again?
> 
> >Well, it's not that simple.  Kernel uses both swappable and
> >non-swappable memory internally.  For some things, it's
> >unswappable, for some, it's swappable.  In general, it's
> >impossible to say which parts of kernel will break (and
> >in wich ways) if swap goes havoc.
> 
> In general, everything you type in as C code (.bss, .data, .text) should be 
> unswappable. kmalloc()ed areas are resident too, and kmalloc has a 
> parameter which defines whether the allocation can/cannot push userspace 
> pages into the swap (GFP_ATOMIC/GFP_IO). So if there is some 
> kernel-allocation swapped out, it is most likely to be marked as 
> 'userspace' so that the same algorithms can be used for swapin and -out.

What are you guys talking about? IIRC kernel doesn't use
swap for its vital data structures. I recall only one
kernel thing which goes into swap: tmpfs data. Caching network
filesystems may also use swappable data, but currently grep
catches only cifs.

IOW swap is for dirtied userspace data. Please correct me
if I am wrong here.
--
vda
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