linux-kernel - Re: [PATCH 5/5] add ksm kernel shared memory driver.

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Date:	Tue, 28 Apr 2009 02:12:00 +0300
From:	Izik Eidus <ieidus@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	kvm@...r.kernel.org, avi@...hat.com, aarcange@...hat.com,
	chrisw@...hat.com, mtosatti@...hat.com, hugh@...itas.com
Subject: Re: [PATCH 5/5] add ksm kernel shared memory driver.

Andrew Morton wrote:
> On Mon, 20 Apr 2009 04:36:06 +0300
> Izik Eidus <ieidus@...hat.com> wrote:
>
>   
>> Ksm is driver that allow merging identical pages between one or more
>> applications in way unvisible to the application that use it.
>> Pages that are merged are marked as readonly and are COWed when any
>> application try to change them.
>>     
>
> Breaks sparc64 and probably lots of other architectures:
>
> mm/ksm.c: In function `try_to_merge_two_pages_alloc':
> mm/ksm.c:697: error: `_PAGE_RW' undeclared (first use in this function)
>
> there should be an official arch-independent way of manipulating
> vma->vm_page_prot, but I'm not immediately finding it.
>   
Hi,

vm_get_page_prot() will probably do the work.

I will send you patch that fix it,
but first i am waiting for Andrea and Chris to say they are happy with 
small changes that i made to the api after conversation i had with them
(about checking if this api is robust enough so we wont have to change 
it later)

When i will get their acks, i will send you patch against this togather 
with the api (until then it is ok to just leave it only for x86)

changes are:
1) limiting the number of memory regions registered per file descriptor 
- so while (1){ (ioctl(KSM_REGISTER_MEMORY_REGION()) ) wont omm the host

2) checking if memory is overlap in registration (more effective to 
ignore such cases)

3) allow removing specific memoy regions inside fd.

Thanks.


> An alternative (and quite inferior) "fix" would be to disable ksm on
> architectures which don't implement _PAGE_RW.  That's most of them.
>
>   

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