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Date:      Sun, 23 Feb 2003 10:35:37 +0000
From:      Christoph Hellwig <hch@infradead.org>
To:        Terry Lambert <tlambert2@mindspring.com>
Cc:        Byunghyun Oh <octphial@postech.ac.kr>, freebsd-hackers@freebsd.org
Subject:   Re: Replacement for get_user_pages() of Linux
Message-ID:  <20030223103537.A16012@infradead.org>
In-Reply-To: <3E589FB3.46C1AAB2@mindspring.com>; from tlambert2@mindspring.com on Sun, Feb 23, 2003 at 02:17:23AM -0800
References:  <20030223163746.A19421@shell.postech.ac.kr> <3E588B1B.484C4D61@mindspring.com> <20030223094958.A15347@infradead.org> <3E589FB3.46C1AAB2@mindspring.com>

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On Sun, Feb 23, 2003 at 02:17:23AM -0800, Terry Lambert wrote:
> OK, you mean "make non-pageable".

Well, I didn't write the initial mail :)

> The question, I guess, is "why?".  Are you trying to do a delayed
> operation that will complete when the process has otherwise been
> swapped out?

well, I don't plan to do anything.  The usual way get_user_pages is used on
linux is:

get_user_pages()

perform scatter gatter dma to some device on the pages

unping pages

as linux VM scanning is fully mutithreaded pages could get paged out
during the dma if you didn't pin them so this is needed.

> > get_user_pages() does not establish a mapping, in Linux you don't need
> > a kernel mapping to perform DMA on memory.
> 
> In FreeBSD, you generally do.  First off, the VM and buffer cache is
> unified.  That means that's there's no such thing as a buffer that
> exists seperately from the VM system.

Who talks about buffers?  And yes, in Linux you can have buffers that
have pages attached to it that aren't mapped into kernel virtual space,
in fact that's usual for pages used for actual filesystem data if you
have enough memory.


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