On Tue, Dec 18, 2018 at 06:24:29PM +0530, Souptick Joarder wrote: > On Tue, Dec 18, 2018 at 6:03 PM Russell King - ARM Linux > <linux@xxxxxxxxxxxxxxx> 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 > > > <linux@xxxxxxxxxxxxxxx> 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); ... and by doing so, you're introducing another instance of the same bug I pointed out in patch 2. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up According to speedtest.net: 11.9Mbps down 500kbps up
