Re: [PATCH v5 00/12] gfs2: Fix mmap + page fault deadlocks

[ Sorry for the delay, I was on the road and this fell through the cracks ]

On Mon, Aug 16, 2021 at 12:14 PM Andreas Gruenbacher
<[email protected]> wrote:
>
> On Tue, Aug 3, 2021 at 9:45 PM Linus Torvalds
> <[email protected]> wrote:
> >
> > Hmm. Have you tried to figure out why that "still returns 0" happens?
>
> The call stack is:
>
> gup_pte_range
> gup_pmd_range
> gup_pud_range
> gup_p4d_range
> gup_pgd_range
> lockless_pages_from_mm
> internal_get_user_pages_fast
> get_user_pages_fast
> iov_iter_get_pages
> __bio_iov_iter_get_pages
> bio_iov_iter_get_pages
> iomap_dio_bio_actor
> iomap_dio_actor
> iomap_apply
> iomap_dio_rw
> gfs2_file_direct_write
>
> In gup_pte_range, pte_special(pte) is true and so we return 0.

Ok, so that is indeed something that the fast-case can't handle,
because some of the special code wants to have the mm_lock so that it
can look at the vma flags (eg "vm_normal_page()" and friends.

That said, some of these cases even the full GUP won't ever handle,
simply because a mapping doesn't necessarily even _have_ a 'struct
page' associated with it if it's a VM_IO mapping.

So it turns out that you can't just always do
fault_in_iov_iter_readable() and then assume that you can do
iov_iter_get_pages() and repeat until successful.

We could certainly make get_user_pages_fast() handle a few more cases,
but I get the feeling that we need to have separate error cases for
EFAULT - no page exists - and the "page exists, but cannot be mapped
as a 'struct page'" case.

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.

                   Linus