From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Wed, 17 Sep 2003 14:24:45 -0400 (EDT) From: Alan Stern Subject: How best to bypass the page cache from within a kernel module? Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org Return-Path: To: linux-mm@kvack.org List-ID: I'm working on a kernel module driver for Linux 2.6. One of the things this driver needs to do is perform a VERIFY command; which means checking to make sure that certain disk sectors within a file actually can be read without encountering a bad sector or other hardware error. Now, I realize that there are already issues involved with convincing the disk drive to read from its media rather than from its cache. But apart from that, my problem is how to convince Linux to read from the drive rather than from the page cache. One suggestion was to use O_DIRECT when opening the file, because that does cause reads to go directly to the hardware. The problem with this is that since the direct-I/O routines send file data directly to user buffers, they must check that the buffer addresses are valid and belong to the user's address space. But my code runs in a kernel thread so it has no current->mm (and in any case I would prefer to use my kernel-space buffers rather than user-space memory). It might be possible to get hold of an mm_struct, but it's not necessarily easy as mm_alloc() isn't EXPORTed. Perhaps my thread could keep its original current->mm by incrementing current->mm->users before calling daemonize() and setting current->mm back to its original value afterward. Is that legal? Having done so, perhaps I could use some sort of mmap() call to allocate a user-space buffer that would be okay for direct-I/O. What's the best way to do that -- what function would I have to call? However, all that seems rather roundabout. An equally acceptable solution would be simply to invalidate all the entries in the page cache referring to my file, so that reads would be forced to go to the drive. Can anyone tell me how to do that? TIA, Alan Stern -- 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