linux-kernel - Re: [PATCH 1/5] staging: zsmalloc: zsmalloc memory allocation library

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Date:	Wed, 08 Feb 2012 15:57:54 -0500
From:	Nitin Gupta <ngupta@...are.org>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
CC:	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Brian King <brking@...ux.vnet.ibm.com>,
	Konrad Wilk <konrad.wilk@...cle.com>, linux-mm@...ck.org,
	devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] staging: zsmalloc: zsmalloc memory allocation library

On 02/08/2012 01:28 PM, Dave Hansen wrote:

> On 02/08/2012 09:53 AM, Nitin Gupta wrote:
>> vmap() is not just slower but also does memory allocations at various
>> places. Under memory pressure, this may cause failure in reading a
>> stored object just because we failed to map it. Also, it allocates VA
>> region each time its called which is a real big waste when we can simply
>> pre-allocate 2 * PAGE_SIZE'ed VA regions (per-cpu).
> 
> Yeah, vmap() is a bit heavy-handed.  I'm just loathe to go mucking
> around in the low-level pagetables too much.  Just seems like there'll
> be a ton of pitfalls, like arch-specific TLB flushing, and it _seems_
> like one of the existing kernel mechanisms should work.
> 
> I guess if you've exhaustively explored all of the existing kernel
> mapping mechanisms and found none of them to work, and none of them to
> be in any way suitably adaptable to your use, you should go ahead and
> roll your own.  I guess you do at least use alloc_vm_area().  What made
> map_vm_area() unsuitable for your needs?  If you're remapping, you
> should at least be guaranteed not to have to allocate pte pages.
> 


map_vm_area() needs 'struct vm_struct' parameter but for mapping kernel
allocated pages within kernel, what should we pass here?  I think we can
instead use map_kernel_range_noflush() -- surprisingly
unmap_kernel_range_noflush() is exported but this one is not.

Nitin

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