linux-kernel - Re: [fuse-devel] [PATCH] FUSE/CUSE: implement direct mmap support

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Date:	Thu, 11 Feb 2010 22:58:19 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Miklos Szeredi <miklos@...redi.hu>
CC:	mszeredi@...e.cz, linux-kernel@...r.kernel.org,
	fuse-devel@...ts.sourceforge.net, polynomial-c@...too.org,
	akpm@...ux-foundation.org
Subject: Re: [fuse-devel] [PATCH] FUSE/CUSE: implement direct mmap support

Hello,

On 02/11/2010 10:40 PM, Miklos Szeredi wrote:
> On Thu, 11 Feb 2010, Tejun Heo wrote:
>>> So there are different offsets:
>>>
>>>  a) vma->vm_pgoff (which may mean anything, but usually means b)
>>
>> Yeap, vma->vm_pgoff can be any value and doesn't really matter.  The
>> only visible difference would be the /proc listing, right?  Setting
>> this to the requested offset is trivial.
> 
> You mean leaving it at the requested offset?  Yes, that's the most
> trivial thing to do.  Very few drivers change vm_pgoff:
> 
>   git grep "vm_pgoff *=[^=]"

Yeap, sure.  I just didn't think it was visible outside.

>>>  b) the offset at which the pages of the mapping are located
>>>  c) the offset at which the server side mmap is located
>>
>> There are three offsets.
>>
>> a) the offset a client requested
>>
>> b) the offset into dmmap AS, a client mmap region is mapped to.  This
>>    could be different from a) by multiple of SHMLBA / PAGE_SIZE.
> 
> No, it could be different from a) by an arbitrary value.

Then, sharing those pages would cause aliasing issues.

>> c) the offset into dmmap AS, a server mmap region is mapped to, where
>>    collection of these mmaps define the dmmap AS.
>>
>> The offsets used in b) and c) are the same offsets.
> 
> Why are they the same?

I meant they point into the same space.  If they're the same value,
they point to the same page.

Thanks.

-- 
tejun
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