From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lf1-f72.google.com (mail-lf1-f72.google.com [209.85.167.72]) by kanga.kvack.org (Postfix) with ESMTP id 429B28E0001 for ; Tue, 18 Dec 2018 07:54:44 -0500 (EST) Received: by mail-lf1-f72.google.com with SMTP id f16so1965209lfc.3 for ; Tue, 18 Dec 2018 04:54:44 -0800 (PST) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id o28sor4211748lfd.61.2018.12.18.04.54.42 for (Google Transport Security); Tue, 18 Dec 2018 04:54:42 -0800 (PST) MIME-Version: 1.0 References: <20181217202334.GA11758@jordon-HP-15-Notebook-PC> <20181218095709.GJ26090@n2100.armlinux.org.uk> <20181218123318.GN26090@n2100.armlinux.org.uk> In-Reply-To: <20181218123318.GN26090@n2100.armlinux.org.uk> From: Souptick Joarder Date: Tue, 18 Dec 2018 18:24:29 +0530 Message-ID: Subject: Re: [PATCH v4 4/9] drm/rockchip/rockchip_drm_gem.c: Convert to use vm_insert_range Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-mm@kvack.org List-ID: To: Russell King - ARM Linux Cc: Michal Hocko , Heiko Stuebner , linux-rockchip@lists.infradead.org, airlied@linux.ie, hjc@rock-chips.com, Matthew Wilcox , linux-kernel@vger.kernel.org, Linux-MM , dri-devel@lists.freedesktop.org, Andrew Morton , linux-arm-kernel@lists.infradead.org On Tue, Dec 18, 2018 at 6:03 PM Russell King - ARM Linux wrote: > > On Tue, Dec 18, 2018 at 05:36:04PM +0530, Souptick Joarder wrote: > > On Tue, Dec 18, 2018 at 3:27 PM Russell King - ARM Linux > > wrote: > > > This looks like a change in behaviour. > > > > > > If user_count is zero, and offset is zero, then we pass into > > > vm_insert_range() a page_count of zero, and vm_insert_range() does > > > nothing and returns zero. > > > > > > However, as we can see from the above code, the original behaviour > > > was to return -ENXIO in that case. > > > > I think these checks are not necessary. I am not sure if we get into mmap > > handlers of driver with user_count = 0. > > I'm not sure either, I'm just pointing out the change of behaviour. Ok. I think feedback from Heiko might be helpful here :) > > > > The other thing that I'm wondering is that if (eg) count is 8 (the > > > object is 8 pages), offset is 2, and the user requests mapping 6 > > > pages (user_count = 6), then we call vm_insert_range() with a > > > pages of rk_obj->pages + 2, and a pages_count of 6 - 2 = 4. So we > > > end up inserting four pages. > > > > Considering the scenario, user_count will remain 8 (user_count = > > vma_pages(vma) ). ? No ? > > Then we call vm_insert_range() with a pages of rk_obj->pages + 2, and > > a pages_count > > of 8 - 2 = 6. So we end up inserting 6 pages. > > > > Please correct me if I am wrong. > > vma_pages(vma) is the number of pages that the user requested, it is > the difference between vma->vm_end and vma->vm_start in pages. As I > said above, "the user requests mapping 6 pages", so vma_pages() will > be 6, and so user_count will also be 6. You are passing > user_count - offset into vm_insert_range(), which will be 6 - 2 = 4 > in my example. This is two pages short of what the user requested. > So, this should be the correct behavior. return vm_insert_range(vma, vma->vm_start, rk_obj->pages + offset, user_count);