REGRESSION: in intel video driver following introduction of mm_struct.has_pinned

[Tony Fischetti]

After a length git bisection, I determined the commit that introduced
a change that ultimately caused a bug/oops null dereference (see below
for relevant syslog entries) was 008cfe4418b3dbda2ff.. (mm: Introduce
mm_struct.has_pinned)

The RIP (according to syslog) occurs in function
`__get_user_pages_remote` and the last function to call it from the
i915 code is `gem_userptr_get_pages_worker`
More specifically, it appears to be the call to
`pin_user_pages_remote` in `gem_userptr_get_pages_worker` in
drivers/gpu/drm/i915/gem/i915_gem_userptr.c that directly leads to the
oops.

Unfortunately, I don't know enough to try to fix and share the fix
myself, but I hope the information I provided is helpful. Please let
me know if there is any further information I can provide that might
be of use.

BUG: kernel NULL pointer dereference, address: 0000000000000054
#PF: supervisor write access in kernel mode
#PF: error_code(0x0002) - not-present page
Oops: 0002 [#1] PREEMPT SMP NOPTI
CPU: 8 PID: 497 Comm: kworker/u25:0 Not tainted
5.9.0-rc7-alice-investigate-3+ #2
Hardware name: LENOVO 10ST001QUS/312A, BIOS M1UKT4BA 11/11/2019
Workqueue: i915-userptr-acquire __i915_gem_userptr_get_pages_worker [i915]
RIP: 0010:__get_user_pages_remote+0xa0/0x2d0
Code: 85 e7 01 00 00 83 3b 01 0f 85 e0 01 00 00 f7 c1 00 00 04 00 0f
84 12 01 00 00 65 48 8b 04 25 00 6d 01 00 48 8b 80 58 03 00 00 <c7> 40
54 01 00 00 00 c6 04 24 00 4d 8d 6f 68 48 c7 44 24 10 00 00
RSP: 0018:ffffa1a58086bde0 EFLAGS: 00010206
RAX: 0000000000000000 RBX: ffffa1a58086be64 RCX: 0000000000040001
RDX: 00000000000007e9 RSI: 00007f532f800000 RDI: ffff92f22d89c480
RBP: 00007f532f800000 R08: ffff92f23a188000 R09: 0000000000000000
R10: 0000000000000000 R11: ffffa1a58086bcfd R12: ffff92f23a188000
R13: ffff92f22d89c480 R14: 0000000000042003 R15: ffff92f22d89c480
FS:  0000000000000000(0000) GS:ffff92f23e400000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000054 CR3: 0000000016c0a002 CR4: 00000000001706e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 __i915_gem_userptr_get_pages_worker+0x1ec/0x392 [i915]
 process_one_work+0x1c7/0x310
 worker_thread+0x28/0x3c0
 ? set_worker_desc+0xb0/0xb0
 kthread+0x123/0x140
 ? kthread_use_mm+0xe0/0xe0
 ret_from_fork+0x1f/0x30
Modules linked in: snd_hda_codec_hdmi snd_hda_codec_realtek
snd_hda_codec_generic ledtrig_audio iwlmvm mac80211 libarc4
x86_pkg_temp_thermal intel_powerclamp iwlwifi coretemp i915
crct10dif_pclmul crc32_pclmul crc32c_intel i2c_algo_bit
ghash_clmulni_intel drm_kms_helper syscopyarea sysfillrect sysimgblt
fb_sys_fops cec mei_hdcp wmi_bmof snd_hda_intel drm tpm_crb
snd_intel_dspcfg intel_wmi_thunderbolt snd_hda_codec snd_hwdep
aesni_intel crypto_simd glue_helper snd_hda_core cfg80211 i2c_i801
snd_pcm intel_cstate pcspkr snd_timer mei_me i2c_smbus mei i2c_core
thermal wmi tpm_tis tpm_tis_core tpm rng_core acpi_pad ppdev lp
ip_tables x_tables
CR2: 0000000000000054
---[ end trace 8d080e8b96289c9e ]---