[Intel-gfx] [PATCH] drm/i915/gem: Avoid iterating an empty list

[Matthew Auld]

On Fri, 22 May 2020 at 13:02, Chris Wilson <chris at chris-wilson.co.uk> wrote:
>
> Quoting Chris Wilson (2020-05-22 11:42:07)
> > Our __sgt_iter assumes that the scattergather list has at least one
> > element. But during construction we may fail in allocating the first
> > page, and so mark the first element as the terminator. This is
> > unexpected!
> >
> > [22555.524752] RIP: 0010:shmem_get_pages+0x506/0x710 [i915]
> > [22555.524759] Code: 49 8b 2c 24 31 c0 66 89 44 24 40 48 85 ed 0f 84 62 01 00 00 4c 8b 75 00 8b 5d 08 44 8b 7d 0c 48 8b 0d 7e 34 07 e2 49 83 e6 fc <49> 8b 16 41 01 df 48 89 cf 48 89 d0 48 c1 e8 2d 48 85 c9 0f 84 c8
> > [22555.524765] RSP: 0018:ffffc9000053f9d0 EFLAGS: 00010246
> > [22555.524770] RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff8881ffffa000
> > [22555.524774] RDX: fffffffffffffff4 RSI: ffffffffffffffff RDI: ffffffff821efe00
> > [22555.524778] RBP: ffff8881b099ab00 R08: 0000000000000000 R09: 00000000fffffff4
> > [22555.524782] R10: 0000000000000002 R11: 00000000ffec0a02 R12: ffff8881cd3c8d60
> > [22555.524786] R13: 00000000fffffff4 R14: 0000000000000000 R15: 0000000000000000
> > [22555.524790] FS:  00007f4fbeb9b9c0(0000) GS:ffff8881f8580000(0000) knlGS:0000000000000000
> > [22555.524795] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > [22555.524799] CR2: 0000000000000000 CR3: 00000001ec7f0004 CR4: 00000000001606e0
> > [22555.524803] Call Trace:
> > [22555.524919]  __i915_gem_object_get_pages+0x4f/0x60 [i915]
> >
>
> Fixes: 85d1225ec066 ("drm/i915: Introduce & use new lightweight SGL iterators")
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Matthew Auld <matthew.auld at intel.com>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: <stable at vger.kernel.org> # v4.8+
Reviewed-by: Matthew Auld <matthew.auld at intel.com>