linux-kernel - Re: [PATCH 1/6] drm/i915: Fix lock order reversal in GTT pwrite path.

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Date:	Fri, 27 Mar 2009 19:35:42 -0700
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Eric Anholt <eric@...olt.net>, linux-kernel@...r.kernel.org,
	dri-devel@...ts.sourceforge.net
Subject: Re: [PATCH 1/6] drm/i915: Fix lock order reversal in GTT pwrite
 path.

On Sat, 28 Mar 2009 01:54:32 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, 2009-03-26 at 17:43 -0700, Jesse Barnes wrote:
> > On Wed, 25 Mar 2009 14:45:05 -0700
> > Eric Anholt <eric@...olt.net> wrote:
> > 
> > > Since the pagefault path determines that the lock order we use
> > > has to be mmap_sem -> struct_mutex, we can't allow page faults to
> > > occur while the struct_mutex is held.  To fix this in pwrite, we
> > > first try optimistically to see if we can copy from user without
> > > faulting.  If it fails, fall back to using get_user_pages to pin
> > > the user's memory, and map those pages atomically when copying it
> > > to the GPU.
> > > 
> > > Signed-off-by: Eric Anholt <eric@...olt.net>
> > > ---
> > > +	/* Pin the user pages containing the data.  We can't
> > > fault while
> > > +	 * holding the struct mutex, and all of the pwrite
> > > implementations
> > > +	 * want to hold it while dereferencing the user data.
> > > +	 */
> > > +	first_data_page = data_ptr / PAGE_SIZE;
> > > +	last_data_page = (data_ptr + args->size - 1) / PAGE_SIZE;
> > > +	num_pages = last_data_page - first_data_page + 1;
> > > +
> > > +	user_pages = kcalloc(num_pages, sizeof(struct page *),
> > > GFP_KERNEL);
> > > +	if (user_pages == NULL)
> > > +		return -ENOMEM;
> > 
> > If kmalloc limits us to a 128k allocation (and maybe less under
> > pressure), then we'll be limited to 128k/8 page pointers on 64 bit,
> > or 64M per pwrite...  Is that ok?  Or do we need to handle multiple
> > passes here?
> 
> While officially supported, a 128k kmalloc is _very_ likely to fail,
> it would require an order 5 page allocation to back that, and that is
> well outside of comfortable.

Yeah, my "and maybe less" could have been worded a tad more strongly. ;)
Do we have stats on which kmalloc buckets have available allocations
anywhere for machines under various workloads?  I know under heavy
pressure even 8k allocations can fail, but since this is a GFP_KERNEL
things should be a *little* better.

-- 
Jesse Barnes, Intel Open Source Technology Center
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