[PATCH 0/3] mm: hugetlbfs: fix hugetlbfs optimization v2

Andrea Arcangeli
2013-11-15T17:48:38Z

Hi,

1/3 is a bugfix so it should be applied more urgently. 1/3 is not as
fast as the current upstream code in the hugetlbfs + directio extreme
8GB/sec benchmark (but 3/3 should fill the gap later). The code is
identical to the one I posted in v1 just rebased on upstream and was
developed in collaboration with Khalid who already tested it.

2/3 and 3/3 had very little testing yet, and they're incremental
optimization. 2/3 is minor and most certainly worth applying later.

3/3 instead complicates things a bit and adds more branches to the THP
fast paths, so it should only be applied if the benchmarks of
hugetlbfs + directio show that it is very worthwhile (that has not
been verified yet). If it's not worthwhile 3/3 should be dropped (and
the gap should be filled in some other way if the gap is not caused by
the _mapcount mangling as I guessed). Ideally this should bring even
more performance than current upstream code, as current upstream code
still increased the _mapcount in gup_fast by mistake, while this
eliminates the locked op on the tail page cacheline in gup_fast too
(which is required for correctness too).

As a side note: the _mapcount refcounting on tail pages is only needed
for THP as it is a fundamental information required for
split_huge_page_refcount to be able to distribute the head refcounts
during the split. And it is done on _mapcount instead of the _count,
because the _count would screwup badly with the get_page_unless_zero
speculative pagecache accesses.

Andrea Arcangeli (3):
mm: hugetlbfs: fix hugetlbfs optimization
mm: hugetlb: use get_page_foll in follow_hugetlb_page
mm: tail page refcounting optimization for slab and hugetlbfs

include/linux/mm.h | 30 ++++++++-
mm/hugetlb.c | 19 +++++-
mm/internal.h | 3 +-
mm/swap.c | 187 ++++++++++++++++++++++++++++++++++-------------------
4 files changed, 170 insertions(+), 69 deletions(-)