Re: [PATCH] drm/i915: Convert WARNs during userptr revoke to SIGBUS

From: Chris Wilson <chris@chris-wilson.co.uk>
To: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH] drm/i915: Convert WARNs during userptr revoke to SIGBUS
Date: Thu, 24 Sep 2015 11:31:17 +0100	[thread overview]
Message-ID: <20150924103117.GT6739@nuc-i3427.alporthouse.com> (raw)
In-Reply-To: <5603CF34.2000208@linux.intel.com>

On Thu, Sep 24, 2015 at 11:23:48AM +0100, Tvrtko Ursulin wrote:
> 
> On 09/23/2015 09:07 PM, Chris Wilson wrote:
> >If the client revokes the virtual address it asked to be mapped into GPU
> >space via userptr (by using anything along the lines of mmap, mprotect,
> >madvise, munmap, ftruncate etc) the mmu notifier sends a range
> >invalidate command to userptr. Upon receiving the invalidation signal
> >for the revoked range, we try to release the struct pages we pinned into
> >the GTT. However, this can fail if any of the GPU's VMA are pinned for
> >use by the hardware (i.e. despite the user's intention we cannot
> >relinquish the client's address range and keep uptodate with whatever is
> >placed in there). Currently we emit a few WARN so that we would notice
> >if this every occurred in the wild; it has. Sadly this means we need to
> >replace those WARNs with the proper SIGBUS to the offending clients
> >instead.
> 
> How does it happen? Frame buffer?

Ignoring the issue of -EIO since patches to fix that path also haven't
landed, the primary cause is through binding the userptr to a scanout
(framebuffer). This is not recommended usage for userptr since the CPU
view is then incoherent, but not impossible.

> >Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> >Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> >Cc: Michał Winiarski <michal.winiarski@intel.com>
> >---
> >  drivers/gpu/drm/i915/i915_gem_userptr.c | 41 +++++++++++++++++++++++++++++----
> >  1 file changed, 37 insertions(+), 4 deletions(-)
> >
> >diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
> >index f75d90118888..efb404b9fe69 100644
> >--- a/drivers/gpu/drm/i915/i915_gem_userptr.c
> >+++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
> >@@ -81,11 +81,44 @@ static void __cancel_userptr__worker(struct work_struct *work)
> 
> This line is a reminder the previous series still hasn't landed. I
> think it was all r-b-ed, with only my request to not rely on
> release_pages (or something) handle negative and zero page count.
> 
> >  		was_interruptible = dev_priv->mm.interruptible;
> >  		dev_priv->mm.interruptible = false;
> >
> >-		list_for_each_entry_safe(vma, tmp, &obj->vma_list, obj_link) {
> >-			int ret = i915_vma_unbind(vma);
> >-			WARN_ON(ret && ret != -EIO);
> >+		list_for_each_entry_safe(vma, tmp, &obj->vma_list, obj_link)
> >+			i915_vma_unbind(vma);
> >+		if (i915_gem_object_put_pages(obj)) {
> >+			struct task_struct *p;
> >+
> >+			DRM_ERROR("Unable to revoke ownership by userptr of"
> >+				  " invalidated address range, sending SIGBUS"
> >+				  " to attached clients.\n");
> >+
> >+			rcu_read_lock();
> >+			for_each_process(p) {
> 
> I don't think this is safe this without holding the tasklist_lock.

Hmm, it's the only lock taken in the oom-killer for sending the signal.
The list will not change nor will tasks disappear whilst we hold the
read-lock so it seems sane.
-Chirs

-- 
Chris Wilson, Intel Open Source Technology Centre
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