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Date:      Wed, 21 Oct 2015 14:13:05 -0700
From:      John Baldwin <jhb@freebsd.org>
To:        src-committers@freebsd.org
Cc:        svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   Re: svn commit: r289719 - head/sys/dev/drm2/i915
Message-ID:  <1544184.b1AHehjrnL@ralph.baldwin.cx>
In-Reply-To: <201510212049.t9LKnkkj002445@repo.freebsd.org>
References:  <201510212049.t9LKnkkj002445@repo.freebsd.org>

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On Wednesday, October 21, 2015 08:49:46 PM John Baldwin wrote:
> Author: jhb
> Date: Wed Oct 21 20:49:45 2015
> New Revision: 289719
> URL: https://svnweb.freebsd.org/changeset/base/289719
> 
> Log:
>   i915_gem_do_execbuffer() holds the pages backing each relocation region for
>   various reasons while executing user commands.  After these commands are
>   completed, the pages backing the relocation regions are unheld.
>   
>   Since relocation regions do not have to be page aligned, the code in
>   validate_exec_list() allocates 2 extra page pointers in the array of
>   held pages populated by vm_fault_quick_hold_pages().  However, the cleanup
>   code that unheld the pages always assumed that only the buffer size /
>   PAGE_SIZE pages were used.  This meant that non-page aligned buffers would
>   not unheld the last 1 or 2 pages in the list.  Fix this by saving the
>   number of held pages returned by vm_fault_quick_hold_pages() for each
>   relocation region and using this count during cleanup.
>   
>   Reviewed by:	dumbbell, kib
>   MFC after:	1 week
>   Differential Revision:	https://reviews.freebsd.org/D3965

I have been getting panics on my x220 due to vm_page_unhold() finding a reference
count on a page of 0 every 2-3 days for the past month or so.  When I examined the
dumps I would find other pages backing the relocation regions that had insanely
high hold counts (like 30 to 60 thousand).  Given that hold_count is a unsigned
short I figured it was overflowing.  The first attempt to fix this was the changes
in r288452.  However, even with that in place I was still seeing the same panics.
I've only run this for a day, but it "looks" correct.  If you have been getting
vm_page_unhold panics with i915kms loaded this is worth trying.  I'm not sure why
my x220 was so prone to this when other people have been able to run X with i915kms
without any problems.  I do have CPUTYPE set to enable AVX, but I don't know that
that would suddenly make lots of buffers non-page aligned that would be page-aligned
otherwise.

-- 
John Baldwin



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