From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Tue, 26 Mar 2002 20:15:02 +0100 From: Andrea Arcangeli Subject: Re: [patch] mmap bug with drivers that adjust vm_start Message-ID: <20020326201502.J13052@dualathlon.random> References: <20020325230046.A14421@redhat.com> <20020326174236.B13052@dualathlon.random> <20020326135703.B25375@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20020326135703.B25375@redhat.com> Sender: owner-linux-mm@kvack.org Return-Path: To: Benjamin LaHaise Cc: Marcelo Tosatti , linux-mm@kvack.org, linux-kernel@vger.kernel.org List-ID: On Tue, Mar 26, 2002 at 01:57:03PM -0500, Benjamin LaHaise wrote: > On Tue, Mar 26, 2002 at 05:42:36PM +0100, Andrea Arcangeli wrote: > > However if the patch is needed it means the ->mmap also must do the > > do_munmap stuff by hand internally, which is very ugly given we also did > > our own do_munmap in a completly different region (the one requested by > > the user). > > At least my own code checks for that and fails if there is a mapping > already placed at the fixed address it needs to use. If we're paranoid, Ok, so it's safe. > we could BUG() on getting a vma back from the new find_vma_prepare call. yes, it sounds a good idea to verify there's no other mapping in the way of the relocation (until a better fix is implemented), it's a slow path so we won't hurt performance. > > > Our do_munmap should not happen if we place the mapping > > elsewhere. If possible I would prefer to change those drivers to > > advertise their enforced vm_start with a proper callback, the current > > way is halfway broken still. BTW, which are those drivers, and why they > > needs to enforce a certain vm_start (also despite MAP_FIXED that they > > cannot check within the ->mmap callback)? > > Video drivers, others that require specific alignment (4MB pages for > example). Historically, the mmap call has been the hook for doing this, > hence the comment in do_mmap from davem. Unless there's a really good > reason for changing the hook, I don't see doing so as providing much > benefit other than making source compatibility hard. The good reason, is that currently we're literally corrupting the userspace with the senseless do_munmap call in the add<->addr+len area before the ->mmap lowlevel callback. And such an munmap is certainly not required to maintain source and binary compatibility (otherwise it would be insane in the first place :). Andrea -- 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/