mm: make faultaround produce old ptes

Based on Kirill's patch [1].

Currently, faultaround code produces young pte.  This can screw up
vmscan behaviour[2], as it makes vmscan think that these pages are hot
and not push them out on first round.

During sparse file access faultaround gets more pages mapped and all of
them are young. Under memory pressure, this makes vmscan swap out anon
pages instead, or to drop other page cache pages which otherwise stay
resident.

Modify faultaround to produce old ptes if sysctl 'want_old_faultaround_pte'
is set, so they can easily be reclaimed under memory pressure.

This can to some extend defeat the purpose of faultaround on machines
without hardware accessed bit as it will not help us with reducing the
number of minor page faults.

Making the faultaround ptes old results in a unixbench regression for some
architectures [3][4]. But on some architectures like arm64 it is not found
to cause any regression.

unixbench shell8 scores on arm64 v8.2 hardware with CONFIG_ARM64_HW_AFDBM
enabled  (5 runs min, max, avg):
Base: (741,748,744)
With this patch: (739,748,743)

So by default produce young ptes and provide a sysctl option to make the
ptes old.

[1] https://marc.info/?l=linux-mm&m=146348837703148
[2] https://lkml.org/lkml/2016/4/18/612
[3] https://marc.info/?l=linux-kernel&m=146582237922378&w=2
[4] https://marc.info/?l=linux-mm&m=146589376909424&w=2



Change-Id: I193185cc953bc33a44fc24963a9df9e555906d95
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Patch-mainline: linux-mm @ Fri, 19 Jan 2018 17:24:54
[vinmenon@codeaurora.org: enable by default since arm works well
with old fault_around ptes + edit the links in commit message to
fix checkpatch issues]
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
[swatsrid@codeaurora.org: Fix merge conflicts]
Signed-off-by: Swathi Sridhar <swatsrid@codeaurora.org>