Re: [RFC][PATCH v2 1/5] mm: Avoid unmapping pinned pages

Peter Zijlstra
2022-01-21T11:41:36Z

On Fri, Jan 21, 2022 at 10:04:45AM +0100, David Hildenbrand wrote:
> On 21.01.22 09:59, Peter Zijlstra wrote:

> > However, I'm not quite sure what fork() does with pages that have a pin.
>
> We COW the anon pages always, and we protect against concurrent GUP
> using the
> * mmap_lock in exclusive mode for ordinary GUP
> * mm->write_protect_seq for GUP-fast

Right, but neither the mmap_sem nor the write_protect_seq help anything
at all vs already extant page pins.

But I just found copy_present_page()'s page_needs_cow_for_dma(), which I
think deals with exactly that case, it avoids doing CoW on pinned pages
and instead feeds the child a full copy while keeping the pinned page in
the original process.

> > Naively, a page that has async DMA activity should not be CoW'ed, or if
> > it is, care must be taken to ensure the original pages stays in the
> > original process, but I realize that's somewhat hard.
>
> That's precisely what I'm working on fixing ... and yes, it's hard.
>
> Let me know if you need any other information, I've spent way too much
> time on this than I ever panned.

So let me try and get this right:

- GUP post-fork breaks CoW for FOLL_WRITE/FOLL_PIN, without either
there's a problem where one task might observe changes by another.

- GUP pre-fork prevents CoW and does a full copy.

And that all mostly works, except for a fair amount of 'fun' cases?