From: Christoph Hellwig <hch@infradead.org>
To: Timur Tabi <timur.tabi@ammasso.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
kernelnewbies@nl.linux.org
Subject: Re: get_user_pages() still broken in 2.6
Date: Wed, 29 Sep 2004 00:03:25 +0100 [thread overview]
Message-ID: <20040929000325.A6758@infradead.org> (raw)
In-Reply-To: <4159E85A.6080806@ammasso.com>; from timur.tabi@ammasso.com on Tue, Sep 28, 2004 at 05:40:26PM -0500
On Tue, Sep 28, 2004 at 05:40:26PM -0500, Timur Tabi wrote:
> I was hoping that this bug would be fixed in the 2.6 kernels, but
> apparently it hasn't been.
>
> Function get_user_pages() is supposed to lock user memory. However,
> under extreme memory constraints, the kernel will swap out the "locked"
> memory.
>
> I have a test app which does this:
>
> 1) Calls our driver, which issues a get_user_pages() call for one page.
> 2) Calls our driver again to get the physical address of that page (the
> driver uses pgd/pmd/pte_offset).
> 3) Tries allocate 1GB of memory (this system has 1GB of physical RAM).
> 4) Tries to get the physical address again.
>
> In step 4, the physical address is usually zero, which means either
> pgd_offset or pmd_offset failed. This indicates the page was swapped out.
>
> I don't understand how this bug can continue to exist after all this
> time. get_user_pages() is supposed to lock the memory, because drivers
> use it for DMA'ing directly into user memory.
get_user_pages locks the page in memory. It doesn't do anything about ptes.
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next prev parent reply other threads:[~2004-09-28 23:03 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-09-28 22:40 Timur Tabi
2004-09-28 23:03 ` Christoph Hellwig [this message]
2004-09-28 23:21 ` Dave Hansen
2004-09-29 14:46 ` Timur Tabi
2004-09-29 14:48 ` Timur Tabi
2004-09-29 15:01 ` Christoph Hellwig
2004-09-29 4:49 ` opening a file inside the kernel module Rakesh Jagota
2004-09-29 5:00 ` Jeff Garzik
2004-09-29 5:56 ` Rakesh Jagota
2004-09-29 15:52 ` Stuart MacDonald, linux-kernel-owner
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