On Thu, Aug 19, 2021 at 10:14 PM Linus Torvalds
[off-list ref] wrote:

On Thu, Aug 19, 2021 at 12:41 PM Andreas Gruenbacher
[off-list ref] wrote:

quoted

Hmm, what if GUP is made to skip VM_IO vmas without adding anything to
the pages array? That would match fault_in_iov_iter_writeable, which
is modeled after __mm_populate and which skips VM_IO and VM_PFNMAP
vmas.

I don't understand what you mean.. GUP already skips VM_IO (and
VM_PFNMAP) pages. It just returns EFAULT.

We could make it return another error. We already have DAX and
FOLL_LONGTERM returning -EOPNOTSUPP.

Of course, I think some code ends up always just returning "number of
pages looked up" and might return 0 for "no pages" rather than the
error for the first page.

So we may end up having interfaces that then lose that explanation
error code, but I didn't check.

But we couldn't make it just say "skip them and try later addresses",
if that is what you meant. THAT makes no sense - that would just make
GUP look up some other address than what was asked for.

get_user_pages has a start and a nr_pages argument, which specifies an
address range from start to start + nr_pages * PAGE_SIZE. If pages !=
NULL, it adds a pointer to that array for each PAGE_SIZE subpage. I
was thinking of skipping over VM_IO vmas in that process, so when the
range starts in a mappable vma, runs into a VM_IO vma, and ends in a
mappable vma, the pages in the pages array would be discontiguous;
they would only cover the mappable vmas. But that would make it
difficult to make sense of what's in the pages array. So scratch that
idea.

quoted

quoted

I also do still think that even regardless of that, we want to just
add a FOLL_NOFAULT flag that just disables calling handle_mm_fault(),
and then you can use the regular get_user_pages().

That at least gives us the full _normal_ page handling stuff.

And it does fix the generic/208 failure.

Good. So I think the approach is usable, even if we might have corner
cases left.

So I think the remaining issue is exactly things like VM_IO and
VM_PFNMAP. Do the fstests have test-cases for things like this? It
_is_ quite specialized, it might be a good idea to have that.

Of course, doing direct-IO from special memory regions with zerocopy
might be something special people actually want to do. But I think
we've had that VM_IO flag testing there basically forever, so I don't
think it has ever worked (for some definition of "ever").

The v6 patch queue should handle those cases acceptably well for now,
but I don't think we have tests covering that at all.

Thanks,
Andreas