From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Wed, 29 Sep 2004 00:03:25 +0100 From: Christoph Hellwig Subject: Re: get_user_pages() still broken in 2.6 Message-ID: <20040929000325.A6758@infradead.org> References: <4159E85A.6080806@ammasso.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4159E85A.6080806@ammasso.com>; from timur.tabi@ammasso.com on Tue, Sep 28, 2004 at 05:40:26PM -0500 Sender: owner-linux-mm@kvack.org Return-Path: To: Timur Tabi Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernelnewbies@nl.linux.org List-ID: 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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: aart@kvack.org