linux-kernel - DIrect-IO using kernel buffers

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Date:	Tue, 17 Oct 2006 10:28:40 +0200
From:	"Ronen Shitrit" <rshitrit@...vell.com>
To:	<linux-kernel@...r.kernel.org>
Cc:	"Linux Memory Management" <linux-mm@...ck.org>
Subject: DIrect-IO using kernel buffers

Hi

I'm using kernel 2.6.12.
I'm trying to improve the usb gadget file storage implementation by
making it accessing the file storage with O_DIRECT,
In general what I'm trying to do is use DirectIO with buffer which was
allocated by kmalloc.

When trying to do so I get kernel panic, after some debug, I found that
I can't call get_user_pages on pages allocated by the kernel (kmalloc),
Is it correct??
Any way, I implemented some basic function get_kernel_pages to fill the
page array with the pages pointer of the kernel buffer,
Then I got lots of errors on illegal buffer freeing, I did some more
debug and I found that the direct-io is using the get_page and put_page
in order to increment the counter of the page and to make sure no one
will release them while the direct-io is using them, after the direct-io
is doing put_page it check if the page_count is 0, if so it release it
(see page_cache_release in direct-io.c), by doing this the direct-io is
trying to release the pages which I allocated by kmalloc and I get
errors.
So what I get is that the direct-io increments the count and then
decrements it and the count get to zero,
So I checked the count of the pages which I got from kmalloc and I found
that only the first page count is 0 (i.e. one process is using it) and
the rest were initialized by -1 (i.e. no process is using this buffer),
Is this a bug??
Any suggestion on how to use DirectIO with kernel buffers??

Regards
Ronen Shitrit

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