Re: [PATCH RFC 0/3] Add dirty-tracking infrastructure for non-page-backed address spaces

[Andy Lutomirski <luto@amacapital.net>]

On 11/19/2013 12:06 PM, Thomas Hellstrom wrote:
> Hi!
> 
> Before going any further with this I'd like to check whether this is an
> acceptable way to go.
> Background:
> GPU buffer objects in general and vmware svga GPU buffers in
> particular are mapped by user-space using MIXEDMAP or PFNMAP. Sometimes the
> address space is backed by a set of pages, sometimes it's backed by PCI memory.
> In the latter case in particular, there is no way to track dirty regions
> using page_mkwrite() and page_mkclean(), other than allocating a bounce
> buffer and perform dirty tracking on it, and then copy data to the real GPU
> buffer. This comes with a big memory- and performance overhead.
> 
> So I'd like to add the following infrastructure with a callback pfn_mkwrite()
> and a function mkclean_mapping_range(). Typically we will be cleaning a range
> of ptes rather than random ptes in a vma.
> This comes with the extra benefit of being usable when the backing memory of
> the GPU buffer is not coherent with the GPU itself, and where we either need
> to flush caches or move data to synchronize.
> 
> So this is a RFC for
> 1) The API. Is it acceptable? Any other suggestions if not?
> 2) Modifying apply_to_page_range(). Better to make a standalone
> non-populating version?
> 3) tlb- mmu- and cache-flushing calls. I've looked at unmap_mapping_range()
> and page_mkclean_one() to try to get it right, but still unsure.

Most (all?) architectures have real dirty tracking -- you can mark a pte
as "clean" and the hardware (or arch code) will mark it dirty when
written, *without* a page fault.

I'm not convinced that it works completely correctly right now (I
suspect that there are some TLB flushing issues on the dirty->clean
transition), and it's likely prone to bit-rot, since the page cache
doesn't rely on it.

That being said, using hardware dirty tracking should be *much* faster
and less latency-inducing than doing it in software like this.  It may
be worth trying to get HW dirty tracking working before adding more page
fault-based tracking.

(I think there's also some oddity on S/390.  I don't know what that
oddity is or whether you should care.)

--Andy

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