Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages

[Jason Gunthorpe]

On Thu, Mar 25, 2021 at 10:51:35AM +0100, Thomas Hellström (Intel) wrote:

> > Please explain that further. Why do we need the mmap lock to insert PMDs
> > but not when insert PTEs?
> 
> We don't. But once you've inserted a PMD directory you can't remove it
> unless you have the mmap lock (and probably also the i_mmap_lock in write
> mode). That for example means that if you have a VRAM region mapped with
> huge PMDs, and then it gets evicted, and you happen to read a byte from it
> when it's evicted and therefore populate the full region with PTEs pointing
> to system pages, you can't go back to huge PMDs again without a munmap() in
> between.

This is all basically magic to me still, but THP does this
transformation and I think what it does could work here too. We
probably wouldn't be able to upgrade while handling fault, but at the
same time, this should be quite rare as it would require the driver to
have supplied a small page for this VMA at some point.

> > Apart from that I still don't fully get why we need this in the first
> > place.
> 
> Because virtual huge page address boundaries need to be aligned with
> physical huge page address boundaries, and mmap can happen before bos are
> populated so you have no way of knowing how physical huge page
> address

But this is a mmap-time problem, fault can't fix mmap using the wrong VA.

> > I really don't see that either. When a buffer is accessed by the CPU it
> > is in > 90% of all cases completely accessed. Not faulting in full
> > ranges is just optimizing for a really unlikely case here.
> 
> It might be that you're right, but are all drivers wanting to use this like
> drm in this respect? Using the interface to fault in a 1G range in the hope
> it could map it to a huge pud may unexpectedly consume and populate some 16+
> MB of page tables.

If the underlying device block size is so big then sure, why not? The
"unexpectedly" should be quite rare/non existant anyhow.

Jason