linux-kernel - Re: [PATCH 0/6] fuse: allocate req->pages[] dynamically

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Date:	Fri, 14 Sep 2012 16:39:44 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	"Maxim V. Patlasov" <mpatlasov@...allels.com>
Cc:	"fuse-devel\@lists.sourceforge.net" 
	<fuse-devel@...ts.sourceforge.net>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel\@openvz.org" <devel@...nvz.org>
Subject: Re: [PATCH 0/6] fuse: allocate req->pages[] dynamically

"Maxim V. Patlasov" <mpatlasov@...allels.com> writes:

> Hi Miklos,
>
> 09/12/2012 08:49 PM, Miklos Szeredi пишет:
>> "Maxim V. Patlasov" <mpatlasov@...allels.com> writes:
>>
>>> Hi Miklos,
>>>
>>> So far as no objections appeared, I'll go ahead and replace fuse req->page with
>>> req->pagevec. It will point to an array of structs:
>>>
>>> struct page_vec {
>>>      struct page    *pv_page;
>>>      unsigned int    pv_len;
>>>      unsigned int    pv_offset;
>>> };
>>>
>>> instead of 'struct page *' as it used to be. It seems to be what you suggested
>>> in one of your comments. Are you OK about it?
>> Yes, that's exactly what I was thinking.
>
> I've encountered a problem while trying to follow this
> approach. fuse_get_user_pages() passes 'req->pages' to
> get_user_pages_fast(). get_user_pages_fast() and friends are not ready to get a
> pointer to array of page_vec-s from fuse. I can see five ways to solve the
> problem:
>
> 1. Re-work get_user_pages_fast() and friends adding ability to fill page_vec
> array. Too much work. Very ugly. I strongly dislike this way.
>
> 2. Allocate a temporary array of page pointers in fuse_get_user_pages() to use
> as argument to get_user_pages_fast(). Ugly and may have performance impact. I
> dislike this way too.
>
> 3. Call get_user_pages_fast() for each page (i.e. pass npages == 1 to it). Easy
> to implement but may have performance impact. I'd refrain from it.
>
> 4. Keep req->pages 'as is', but add req->page_descs pointing to an array of
> <offset, len> structures. Looks clumsy, straightforward, but quite
> doable.
>
> 5. Use a hack in fuse_get_user_pages(): temporarily cast req->pagevecs to
> struct page **pages', pass it get_user_pages_fast(), then transform the content
> of req->pagevecs[] to have page pointers stored in proper places (like 'for
> (i=...) pagevecs[i].pv_page = pages[i];').
>
> What do you think?

I'd go for number 4.


Thanks,
Miklos
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