On Wed, Aug 20, 2014 at 08:56:09AM -0600, Dann Frazier wrote:

On Wed, Jun 25, 2014 at 9:40 AM, Steve Capper [off-list ref] wrote:

quoted

Hello,
This series implements general forms of get_user_pages_fast and
__get_user_pages_fast and activates them for arm and arm64.

These are required for Transparent HugePages to function correctly, as
a futex on a THP tail will otherwise result in an infinite loop (due to
the core implementation of __get_user_pages_fast always returning 0).

This series may also be beneficial for direct-IO heavy workloads and
certain KVM workloads.

The main changes since RFC V5 are:
 * Rebased against 3.16-rc1.
 * pmd_present no longer tested for by gup_huge_pmd and gup_huge_pud,
   because the entry must be present for these leaf functions to be
   called.
 * Rather than assume puds can be re-cast as pmds, a separate
   function pud_write is instead used by the core gup.
 * ARM activation logic changed, now it will only activate
   RCU_TABLE_FREE and RCU_GUP when running with LPAE.

The main changes since RFC V4 are:
 * corrected the arm64 logic so it now correctly rcu-frees page
   table backing pages.
 * rcu free logic relaxed for pre-ARMv7 ARM as we need an IPI to
   invalidate TLBs anyway.
 * rebased to 3.15-rc3 (some minor changes were needed to allow it to merge).
 * dropped Catalin's mmu_gather patch as that's been merged already.

This series has been tested with LTP and some custom futex tests that
exacerbate the futex on THP tail case. Also debug counters were
temporarily employed to ensure that the RCU_TABLE_FREE logic was
behaving as expected.

I would really appreciate any testers or comments (especially on the
validity or otherwise of the core fast_gup implementation).

I have a test case that can reliably hit the THP issue on arm64, which
hits it on both 3.16 and 3.17-rc1. I do a "juju bootstrap local" w/
THP disabled at boot. Then I reboot with THP enabled. At this point
you'll see jujud spin at 200% CPU. gccgo binaries seem to have a nack
for hitting it.

I validated that your patches resolve this issue on 3.16, so:

Tested-by: dann frazier <redacted>

Thanks Dann!

I haven't done the same for 3.17-rc1 because they no longer apply
cleanly, but I'm happy to test future submissions w/ hopefully a
shorter feedback loop (please add me to the CC). btw, should we
consider something like this until your patches go in?

I am about to post the following series, I will CC you:
git://git.linaro.org/people/steve.capper/linux.git fast_gup/3.17-rc1
(I've just been giving it a workout on 3.17-rc1).

I would much prefer for the RCU fast_gup to go into 3.18 rather than
BROKEN for THP. I am not sure what to do about earlier versions.

Cheers,
-- 
Steve

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