Re: [PATCH 1/4] mm: Trial do_wp_page() simplification

Oded Gabbay
2022-02-16T19:21:23Z

On Wed, Feb 16, 2022 at 9:04 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> [ Added David Hildenbrand to the participants. David, see
>
> https://bugzilla.kernel.org/show_bug.cgi?id=215616
>
> for details ]
>
> On Wed, Feb 16, 2022 at 8:59 AM Oded Gabbay <oded.gabbay@gmail.com> wrote:
> >
> > All the details are in the bug, but the bottom line is that somehow,
> > this patch causes corruption when the numa balancing feature is
> > enabled AND we don't use process affinity AND we use GUP to pin pages
> > so our accelerator can DMA to/from system memory.
>
> Hmm. I thought all the remaining issues were related to THP - and
> David Hildenbrand had a series to fix those up.
>
> The fact that it also shows up with numa balancing is a bit
> unfortunate, because I think that means that that patch series may not
> have caught that case.
>
> That said - what does "we use GUP to pin pages" mean? Does it actually
> use the pinning logic, or just regular old GUP?
>
> I'm assuming this is just the existing pin_user_pages_fast() (ie a
> proper pin) in drivers/misc/habanalabs/common/memory.c. But I wanted
> to confirm that it's not some other situation.
>
> Linus
>
[Adding dave hansen as we chatted about it in irc]

It uses the pinning logic, simply calling pin_user_pages_fast with the
relevant gup flags to pin the userspace memory so we can after that
dma map it and give the bus address to the h/w.

And afaik, the gup flags combination we use
(FOLL_FORCE | FOLL_WRITE | FOLL_LONGTERM) is the correct combination,
at least according to the last time it was discussed with Daniel,
Jason and other people.

Oded