bufmap: manage as folios, V2.

    Thanks for the feedback from Dan Carpenter and Arnd Bergmann.

       Dan suggested to make the rollback loop in orangefs_bufmap_map
       more robust.

       Arnd caught a %ld format for a size_t in
       orangefs_bufmap_copy_to_iovec. He suggested %zd, I
       used %zu which I think is OK too.

    Orangefs userspace allocates 40 megabytes on an address that's page
    aligned.

    With this folio modification the allocation is aligned on a multiple of
    2 megabytes:
    posix_memalign(&ptr, 2097152, 41943040);

    Then userspace tries to enable Huge Pages for the range:
    madvise(ptr, 41943040, MADV_HUGEPAGE);

    Userspace provides the address of the 40 megabyte allocation to
    the Orangefs kernel module with an ioctl.

    The kernel module initializes the memory as a "bufmap" with ten
    4 megabyte "slots".

    Traditionally, the slots are manipulated a page at a time.

    This folio/bufmap modification manages the slots as folios, with
    two 2 megabyte folios per slot and data can be read into
    and out of each slot a folio at a time.

    This modification works fine with orangefs userspace lacking
    the THP focused posix_memalign and madvise settings listed above,
    each slot can end up being made of page sized folios. It also works
    if there are some, but less than 20, hugepages available. A message
    is printed in the kernel ring buffer (dmesg) at userspace start
    time that describes the folio/page ratio. As an example, I started
    orangefs and saw "Grouped 2575 folios from 10240 pages" in the ring
    buffer.

    To get the optimum ratio, 20/10240, I use these settings before
    I start the orangefs userspace:

      echo always > /sys/kernel/mm/transparent_hugepage/enabled
      echo always > /sys/kernel/mm/transparent_hugepage/defrag
      echo 30 > /proc/sys/vm/nr_hugepages

    https://docs.kernel.org/admin-guide/mm/hugetlbpage.html discusses
    hugepages and manipulating the /proc/sys/vm settings.

    Comparing the performance between the page/bufmap and the folio/bufmap
    is a mixed bag.

      - The folio/bufmap version is about 8% faster at running through the
        xfstest suite on my VMs.

       - It is easy to construct an fio test that brings the page/bufmap
         version to its knees on my dinky VM test system, with all bufmap
         slots used and I/O timeouts cascading.

       - Some smaller tests I did with fio that didn't overwhelm the
         page/bufmap version showed no performance gain with the
         folio/bufmap version on my VM.

    I suspect this change will improve performance only in some use-cases.
    I think it will be a gain when there are many concurrent IOs that
    mostly fill the bufmap. I'm working up a gcloud test for that.

    Reported-by: Dan Carpenter <error27@gmail.com>
    Reported-by: Arnd Bergmann <arnd@arndb.de>
    Signed-off-by: Mike Marshall <hubcap@omnibond.com>

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