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

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Date:	Wed, 08 Feb 2012 12:53:40 -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 11:39 AM, Dave Hansen wrote:

> On 02/06/2012 09:26 AM, Seth Jennings wrote:
>> On 01/26/2012 01:12 PM, Dave Hansen wrote:
>>> void *kmap_atomic_prot(struct page *page, pgprot_t prot)
>>> {
>>> ...
>>>         type = kmap_atomic_idx_push();
>>>         idx = type + KM_TYPE_NR*smp_processor_id();
>>>         vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);
>>>
>>> I think if you do a get_cpu()/put_cpu() or just a preempt_disable()
>>> across the operations you'll be guaranteed to get two contiguous addresses.
>>
>> I'm not quite following here.  kmap_atomic() only does this for highmem pages.
>> For normal pages (all pages for 64-bit), it doesn't do any mapping at all.  It
>> just returns the virtual address of the page since it is in the kernel's address
>> space.
>>
>> For this design, the pages _must_ be mapped, even if the pages are directly
>> reachable in the address space, because they must be virtually contiguous.
> 
> I guess you could use vmap() for that.  It's just going to be slower
> than kmap_atomic().  I'm really not sure it's worth all the trouble to
> avoid order-1 allocations, though.
> 


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).

Thanks,
Nitin
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