Re: [PATCH] libnvdimm/pmem: Bypass CONFIG_HARDENED_USERCOPY overhead

[Dan Williams <dan.j.williams@intel.com>]

On Fri, May 17, 2019 at 1:47 AM Jan Kara <jack@suse.cz> wrote:
>
> Let's add Kees to CC for usercopy expertise...
>
> On Thu 16-05-19 17:33:38, Dan Williams wrote:
> > Jeff discovered that performance improves from ~375K iops to ~519K iops
> > on a simple psync-write fio workload when moving the location of 'struct
> > page' from the default PMEM location to DRAM. This result is surprising
> > because the expectation is that 'struct page' for dax is only needed for
> > third party references to dax mappings. For example, a dax-mapped buffer
> > passed to another system call for direct-I/O requires 'struct page' for
> > sending the request down the driver stack and pinning the page. There is
> > no usage of 'struct page' for first party access to a file via
> > read(2)/write(2) and friends.
> >
> > However, this "no page needed" expectation is violated by
> > CONFIG_HARDENED_USERCOPY and the check_copy_size() performed in
> > copy_from_iter_full_nocache() and copy_to_iter_mcsafe(). The
> > check_heap_object() helper routine assumes the buffer is backed by a
> > page-allocator DRAM page and applies some checks.  Those checks are
> > invalid, dax pages are not from the heap, and redundant,
> > dax_iomap_actor() has already validated that the I/O is within bounds.
>
> So this last paragraph is not obvious to me as check_copy_size() does a lot
> of various checks in CONFIG_HARDENED_USERCOPY case. I agree that some of
> those checks don't make sense for PMEM pages but I'd rather handle that by
> refining check_copy_size() and check_object_size() functions to detect and
> appropriately handle pmem pages rather that generally skip all the checks
> in pmem_copy_from/to_iter(). And yes, every check in such hot path is going
> to cost performance but that's what user asked for with
> CONFIG_HARDENED_USERCOPY... Kees?

As far as I can see it's mostly check_heap_object() that is the
problem, so I'm open to finding a way to just bypass that sub-routine.
However, as far as I can see none of the other block / filesystem user
copy implementations submit to the hardened checks, like
bio_copy_from_iter(), and iov_iter_copy_from_user_atomic() . So,
either those need to grow additional checks, or the hardened copy
implementation is targeting single object copy use cases, not
necessarily block-I/O. Yes, Kees, please advise.