nouveau. swiotlb: coherent allocation failed for device 0000:01:00.0 size=2097152

[Mike Galbraith]

Greetings,

Kernel bound workloads seem to trigger the below for whatever reason.
 I only see this when beating up NFS.  There was a kworker wakeup
latency issue, but with a bandaid applied to fix that up, I can still
trigger this.

[ 1313.811031] nouveau 0000:01:00.0: swiotlb buffer is full (sz: 2097152 bytes)
[ 1313.811035] swiotlb: coherent allocation failed for device 0000:01:00.0 size=2097152
[ 1313.811038] CPU: 6 PID: 3026 Comm: Xorg Tainted: G            E    4.15.0.g1291a0d5-master #355
[ 1313.811040] Hardware name: MEDION MS-7848/MS-7848, BIOS M7848W08.20C 09/23/2013
[ 1313.811041] Call Trace:
[ 1313.811049]  dump_stack+0x7c/0xb6
[ 1313.811053]  swiotlb_alloc_coherent+0x13f/0x150
[ 1313.811060]  ttm_dma_pool_alloc_new_pages+0x106/0x3c0 [ttm]
[ 1313.811066]  ttm_dma_pool_get_pages+0x10a/0x1e0 [ttm]
[ 1313.811070]  ttm_dma_populate+0x21f/0x2f0 [ttm]
[ 1313.811075]  ttm_tt_bind+0x2f/0x60 [ttm]
[ 1313.811079]  ttm_bo_handle_move_mem+0x51f/0x580 [ttm]
[ 1313.811084]  ? ttm_bo_handle_move_mem+0x5/0x580 [ttm]
[ 1313.811088]  ttm_bo_validate+0x10c/0x120 [ttm]
[ 1313.811092]  ? ttm_bo_validate+0x5/0x120 [ttm]
[ 1313.811106]  ? drm_mode_setcrtc+0x20e/0x540 [drm]
[ 1313.811109]  ttm_bo_init_reserved+0x290/0x490 [ttm]
[ 1313.811114]  ttm_bo_init+0x52/0xb0 [ttm]
[ 1313.811141]  ? nv10_bo_put_tile_region+0x60/0x60 [nouveau]
[ 1313.811163]  nouveau_bo_new+0x465/0x5e0 [nouveau]
[ 1313.811184]  ? nv10_bo_put_tile_region+0x60/0x60 [nouveau]
[ 1313.811203]  nouveau_gem_new+0x66/0x110 [nouveau]
[ 1313.811223]  ? nouveau_gem_new+0x110/0x110 [nouveau]
[ 1313.811241]  nouveau_gem_ioctl_new+0x48/0xc0 [nouveau]
[ 1313.811249]  drm_ioctl_kernel+0x64/0xb0 [drm]
[ 1313.811257]  drm_ioctl+0x2a4/0x360 [drm]
[ 1313.811276]  ? nouveau_gem_new+0x110/0x110 [nouveau]
[ 1313.811285]  ? drm_ioctl+0x5/0x360 [drm]
[ 1313.811304]  nouveau_drm_ioctl+0x50/0xb0 [nouveau]
[ 1313.811308]  do_vfs_ioctl+0x90/0x690
[ 1313.811311]  ? do_vfs_ioctl+0x5/0x690
[ 1313.811313]  SyS_ioctl+0x3b/0x70
[ 1313.811316]  entry_SYSCALL_64_fastpath+0x1f/0x91
[ 1313.811320] RIP: 0033:0x7f3234746227
[ 1313.811321] RSP: 002b:00007ffc3ace0408 EFLAGS: 00003246 ORIG_RAX: 0000000000000010
[ 1313.811324] RAX: ffffffffffffffda RBX: 00000000025515d0 RCX: 00007f3234746227
[ 1313.811325] RDX: 00007ffc3ace0460 RSI: 00000000c0306480 RDI: 000000000000000b
[ 1313.811326] RBP: 0000000000824120 R08: 0000000002548f80 R09: 00000000025490d0
[ 1313.811328] R10: 0000000000000000 R11: 0000000000003246 R12: 000000000000093d
[ 1313.811329] R13: 0000000002aff74c R14: 0000000000824150 R15: 0000000000000000