linux-kernel - Re: [PATCH 3/4] zsmalloc: add details to zs_map

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Date:	Thu, 12 Jul 2012 10:15:55 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Nitin Gupta <ngupta@...are.org>,
	Robert Jennings <rcj@...ux.vnet.ibm.com>, linux-mm@...ck.org,
	devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] zsmalloc: add details to zs_map_object boiler plate

On Wed, Jul 11, 2012 at 09:15:43AM -0500, Seth Jennings wrote:
> On 07/11/2012 02:42 AM, Minchan Kim wrote:
> > On 07/11/2012 12:17 AM, Seth Jennings wrote:
> >> On 07/09/2012 09:35 PM, Minchan Kim wrote:
> >>> Maybe we need local_irq_save/restore in zs_[un]map_object path.
> >>
> >> I'd rather not disable interrupts since that will create
> >> unnecessary interrupt latency for all users, even if they
> > 
> > Agreed.
> > Although we guide k[un]map atomic is so fast, it isn't necessary
> > to force irq_[enable|disable]. Okay.
> > 
> >> don't need interrupt protection.  If a particular user uses
> >> zs_map_object() in an interrupt path, it will be up to that
> >> user to disable interrupts to ensure safety.
> > 
> > Nope. It shouldn't do that.
> > Any user in interrupt context can't assume that there isn't any other user using per-cpu buffer
> > right before interrupt happens.
> > 
> > The concern is that if such bug happens, it's very hard to find a bug.
> > So, how about adding this?
> > 
> > void zs_map_object(...)
> > {
> > 	BUG_ON(in_interrupt());
> > }
> 
> I not completely following you, but I think I'm following
> enough.  Your point is that the per-cpu buffers are shared
> by all zsmalloc users and one user doesn't know if another
> user is doing a zs_map_object() in an interrupt path.

And vise versa is yes.

> 
> However, I think what you are suggesting is to disallow
> mapping in interrupt context.  This is a problem for zcache
> as it already does mapping in interrupt context, namely for
> page decompression in the page fault handler.

I don't get it.
Page fault handler isn't interrupt context.

> 
> What do you think about making the per-cpu buffers local to
> each zsmalloc pool? That way each user has their own per-cpu
> buffers and don't step on each other's toes.

Maybe, It could be a solution if you really need it in interrupt context.
But the concern is it could hurt zsmalloc's goal which is memory
space efficiency if your system has lots of CPUs.

> 
> Thanks,
> Seth
> 
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