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

[Andreas Gruenbacher <agruenba@redhat.com>]

On Tue, Aug 3, 2021 at 9:45 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Tue, Aug 3, 2021 at 12:18 PM Andreas Gruenbacher <agruenba@redhat.com> wrote:
> > With this patch queue, fstest generic/208 (aio-dio-invalidate-failure.c)
> > endlessly spins in gfs2_file_direct_write.  It looks as if there's a bug
> > in get_user_pages_fast when called with FOLL_FAST_ONLY:
> >
> >  (1) The test case performs an aio write into a 32 MB buffer.
> >
> >  (2) The buffer is initially not in memory, so when iomap_dio_rw() ->
> >      ... -> bio_iov_iter_get_pages() is called with the iter->noio flag
> >      set, we get to get_user_pages_fast() with FOLL_FAST_ONLY set.
> >      get_user_pages_fast() returns 0, which causes
> >      bio_iov_iter_get_pages to return -EFAULT.
> >
> >  (3) Then gfs2_file_direct_write faults in the entire buffer with
> >      fault_in_iov_iter_readable(), which succeeds.
> >
> >  (4) With the buffer in memory, we retry the iomap_dio_rw() ->
> >      ... -> bio_iov_iter_get_pages() -> ... -> get_user_pages_fast().
> >      This should succeed now, but get_user_pages_fast() still returns 0.
>
> 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.

> One option - for debugging only - would be to introduce a new flag to
> get_user_pages_fast() that says "print out reason if failed" and make
> the retry (but not the original one) have that flag set.
>
> There are a couple of things of note when it comes to "get_user_pages_fast()":
>
>  (a) some architectures don't even enable it
>
>  (b) it can be very picky about the page table contents, and wants the
> accessed bit to already be set (or the dirty bit, in the case of a
> write).
>
> but (a) shouldn't be an issue on any common platform and (b) shouldn't
> be an issue with  fault_in_iov_iter_readable() that actually does a
> __get_user() so it will access through the page tables.
>
> (It might be more of an issue with fault_in_iov_iter_writable() due to
> walking the page tables by hand - if we don't do the proper
> access/dirty setting, I could see get_user_pages_fast() failing).
>
> Anyway, for reason (a) I do think that eventually we should probably
> introduce FOLL_NOFAULT, and allow the full "slow" page table walk -
> just not calling down to handle_mm_fault() if it fails.
>
> But (a) should be a non-issue in your test environment, and so it
> would be interesting to hear what it is that fails. Because scanning
> through the patches, they all _look_ fine to me (apart from the one
> comment about return values, which is more about being consistent with
> copy_to/from_user() and making the code simpler - not about
> correctness)
>
>                        Linus
>

Thanks,
Andreas