Why can't ttm_tt_swapout() handle uncached or WC BOs?

[Hellstrom, Thomas]

On Wed, 2020-09-23 at 16:03 +0200, Christian König wrote:
> Am 23.09.20 um 11:24 schrieb Hellstrom, Thomas:
> > On Wed, 2020-09-23 at 13:17 +1000, Dave Airlie wrote:
> > > On Fri, 18 Sep 2020 at 00:49, Christian König <
> > > christian.koenig at amd.com> wrote:
> > > > Am 17.09.20 um 16:44 schrieb Michel Dänzer:
> > > > > On 2020-09-17 2:20 p.m., Christian König wrote:
> > > > > > Hi guys,
> > > > > > 
> > > > > > Michel once submitted a patch to fix triggering this BUG_ON
> > > > > > in
> > > > > > ttm_tt_swapout():
> > > > > > 
> > > > > > > BUG_ON(ttm->caching_state != tt_cached);
> > > > > > Now my question is does anybody know why we have that in
> > > > > > the
> > > > > > first
> > > > > > place?
> > > > > > 
> > > > > > The only problematic thing I can see is calling
> > > > > > copy_highpage()
> > > > > > and
> > > > > > that one is just doing a kmap_atomic()/kunmap_atomic() on
> > > > > > the
> > > > > > source
> > > > > > and destination.
> > > > > > 
> > > > > > I can't see why it should be problematic for this temporary
> > > > > > mapping
> > > > > > to be cached instead of uncached or WC?
> > > > > > 
> > > > > > Does anybody has any idea?
> > > > > One thing is that AFAIK some (ARM?) CPUs can get very upset
> > > > > when
> > > > > there's both a cached and uncacheable mapping for the same
> > > > > physical page.
> > > > Good point, but I already considered this.
> > > > 
> > > > If there is another mapping for that page left we are
> > > > completely
> > > > busted
> > > > anyway since we are currently changing the underlying storage.
> > > > 
> > > It's not just ARM of course, x86 PAT also has many issues about
> > > multiple mappings of the same backing page with different
> > > attributes.
> > > 
> > > The only mappings might be in the linear mapping, since not all
> > > pages
> > > we get here will necessarily be high pages I assume and therefore
> > > could have a linear mapping. If we've changed the linear mapping
> > > to
> > > non-cached, then create a cached kmap I'm not 100% that won't
> > > cause
> > > issues.
> > > 
> > > but this is a definite maze of twisty passages and I'd probably
> > > need
> > > to sit down and break it a bit more.
> > > 
> > > Dave.
> > This is a problem that goes back way far (12+) years that the x86
> > architecture is not allowed to do aliased mappings of pages, even
> > through mappable GTTs: There are two aspects:
> > 
> > 1) Create a WC mapping of a page with data in the cache. When the
> > cache
> > does a writeback, anything written through the WC mapping will get
> > overwritten.
> > 
> > 2) Flushing the WB mappings first might not help, since at that
> > time
> > there were some AMD processors (Athlons?) that were speculatively
> > prefetching data on the WB mapping into the cache at any time, and
> > even
> > though it wasn't changed it did a writeback. Anything written
> > through
> > WC in-between the prefetch and the writeback got overwritten. That
> > was
> > a real and occuring problem at that time. AMD claimed it was not
> > violating specs.
> > 
> > So aliased mappings is probably at best very fragile.
> 
> All of this is correct, but I still can't see why ttm_bo_swapout()
> tries 
> to change the caching?
> 
> See copy_highpage() will only create a temporary wb mapping for
> highmem 
> pages which is destroyed immediately again. Otherwise it will use
> the 
> linear mapping which should already have the correct caching
> attributes.
> 

IIRC there are a couple of historical reasons
- Generally avoiding aliased mappings with conflicting caching
- Reading out from WC is dead slow
- Not sure of the order of events, but I think swapping predates the WC
page pool, which meant the pages had to change caching anyway.
- A desire to be able insert the pages directly in the swap cache
rather than to copy shmem objects.

But today, we can probably do better. I guess, where available,
streaming WC-tailored memcpy and not changing the caching might be a
good choice?

/Thomas







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