Re: [PATCH v2 3/3] mm: perform the mapping_map_writable() check after call_mmap()

[Lorenzo Stoakes]

On Tue, May 16, 2023 at 01:52:06PM +0800, kernel test robot wrote:
> Hello,
>
> kernel test robot noticed "assertion_failure" on:
>
> commit: a0e22a91f487957346732c6613eb6bd1b7c72ab1 ("[PATCH v2 3/3] mm: perform the mapping_map_writable() check after call_mmap()")
> url: https://github.com/intel-lab-lkp/linux/commits/Lorenzo-Stoakes/mm-drop-the-assumption-that-VM_SHARED-always-implies-writable/20230501-062815
> base: https://git.kernel.org/cgit/linux/kernel/git/akpm/mm.git mm-everything
> patch link: https://lore.kernel.org/all/6f3aea05c9cc46094b029cbd1138d163c1ae7f9d.1682890156.git.lstoakes@xxxxxxxxx/
> patch subject: [PATCH v2 3/3] mm: perform the mapping_map_writable() check after call_mmap()
>
> in testcase: igt
> version: igt-x86_64-9e9cd7e6-1_20230506
> with following parameters:
>
> 	group: group-11
>
> compiler: gcc-11
> test machine: 20 threads 1 sockets (Commet Lake) with 16G memory
>
> (please refer to attached dmesg/kmsg for entire log/backtrace)
>
>
> If you fix the issue, kindly add following tag
> | Reported-by: kernel test robot <yujie.liu@xxxxxxxxx>
> | Link: https://lore.kernel.org/oe-lkp/202305161044.bba89e76-yujie.liu@xxxxxxxxx
>
>
> 2023-05-11 12:29:38 build/tests/gem_mmap_gtt --run-subtest basic-copy
> IGT-Version: 1.27.1-g9e9cd7e6 (x86_64) (Linux: 6.3.0-10673-ga0e22a91f487 x86_64)
> Starting subtest: basic-copy
> (gem_mmap_gtt:1138) i915/gem_mman-CRITICAL: Test assertion failure function gem_mmap__gtt, file ../lib/i915/gem_mman.c:146:
> (gem_mmap_gtt:1138) i915/gem_mman-CRITICAL: Failed assertion: ptr
> (gem_mmap_gtt:1138) i915/gem_mman-CRITICAL: Last errno: 1, Operation not permitted
> Subtest basic-copy failed.
[snip]

I don't have the hardware to test this (the repro steps don't work and
manually running the test indicates the actual hardware is required) but I
suspect it's a result of i915_gem_mmap() somehow causing
mapping_unmap_writable() to be invoked, which sets mapping->i_mmap_writable
negative, and thus the check after call_mmap() is performed reports the error.

In v3 I will change this to continue to mark the file writable before
invoking call_mmap() which should fix this issue.