On Thu 07-01-21 13:53:18, John Hubbard wrote:

On 1/7/21 1:29 PM, Linus Torvalds wrote:

quoted

On Thu, Jan 7, 2021 at 12:59 PM Andrea Arcangeli [off-list ref] wrote:

quoted

The problem is it's not even possible to detect reliably if there's
really a long term GUP pin because of speculative pagecache lookups.

So none of the normal code _needs_ that any more these days, which is
what I think is so nice. Any pinning will do the COW, and then we have
the logic to make sure it stays writable, and that keeps everything
nicely coherent and is all fairly simple.

And yes, it does mean that if somebody then explicitly write-protects
a page, it may end up being COW'ed after all, but if you first pinned
it, and then started playing with the protections of that page, why
should you be surprised?

So to me, this sounds like a "don't do that then" situation.

Anybody who does page pinning and wants coherency should NOT TOUCH THE
MAPPING IT PINNED.

(And if you do touch it, it's your own fault, and you get to keep both
of the broken pieces)

Now, I do agree that from a QoI standpoint, it would be really lovely
if we actually enforced it. I'm not entirely sure we can, but maybe it
would be reasonable to use that

   mm->has_pinned && page_maybe_dma_pinned(page)

at least as the beginning of a heuristic.

In fact, I do think that "page_maybe_dma_pinned()" could possibly be
made stronger than it is. Because at *THAT* point, we might say "we

What exactly did you have in mind, to make it stronger? I think the
answer is in this email but I don't quite see it yet...

Also, now seems to be a good time to mention that I've been thinking about
a number of pup/gup pinning cases (Direct IO, GPU/NIC, NVMe/storage peer
to peer with GUP/NIC, and HMM support for atomic operations from a device).
And it seems like the following approach would help:

* Use pin_user_pages/FOLL_PIN for long-term pins. Long-term here (thanks
to Jason for this point) means "user space owns the lifetime". We might
even end up deleting either FOLL_PIN or FOLL_LONGTERM, because this would
make them mean the same thing. The idea is that there are no "short term"
pins of this kind of memory.

* Continue to use FOLL_GET (only) for Direct IO. That's a big change of plans,
because several of us had thought that Direct IO needs FOLL_PIN. However, this
recent conversation, plus my list of cases above, seems to indicate otherwise.
That's because we only have one refcount approach for marking pages in this way,
and we should spend it on the long-term pinned pages. Those are both hard to
identify otherwise, and actionable once we identify them.

Somewhat late to the game but I disagree here. I think direct IO still
needs FOLL_PIN so that page_may_be_dma_pinned() returns true for it. At
least for shared pages. Because filesystems/mm in the writeback path need
to detect whether the page is pinned and thus its contents can change
anytime without noticing, the page can be dirtied at random times etc. In
that case we need to bounce the page during writeback (to avoid checksum
failures), keep page as dirty in internal filesystem bookkeeping (and in MM
as well) etc...

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR