[PATCH] CMA/HOTPLUG: clear buffer-head lru before page migration

[Gioh Kim <gioh.kim@lge.com>]

Hi,

For page migration of CMA, buffer-heads of lru should be dropped.
Please refer to https://lkml.org/lkml/2014/7/4/101 for the history.

I have two solution to drop bhs.
One is invalidating entire lru.
Another is searching the lru and dropping only one bh that Laura proposed
at https://lkml.org/lkml/2012/8/31/313.

I'm not sure which has better performance.
So I did performance test on my cortex-a7 platform with Lmbench
that has "File & VM system latencies" test.
I am attaching the results.
The first line is of invalidating entire lru and the second is dropping selected bh.

File & VM system latencies in microseconds - smaller is better
-------------------------------------------------------------------------------
Host                 OS   0K File      10K File     Mmap    Prot   Page   100fd
                        Create Delete Create Delete Latency Fault  Fault  selct
--------- ------------- ------ ------ ------ ------ ------- ----- ------- -----
10.178.33 Linux 3.10.19   25.1   19.6   32.6   19.7  5098.0 0.666 3.45880 6.506
10.178.33 Linux 3.10.19   24.9   19.5   32.3   19.4  5059.0 0.563 3.46380 6.521


I tried several times but the result tells that they are the same under 1% gap
except Protection Fault.
But the latency of Protection Fault is very small and I think it has little effect.

Therefore we can choose anything but I choose invalidating entire lru.
The try_to_free_buffers() which is calling drop_buffers() is called by many filesystem code.
So I think inserting codes in drop_buffers() can affect the system.
And also we cannot distinguish migration type in drop_buffers().

In alloc_contig_range() we can distinguish migration type and invalidate lru if it needs.
I think alloc_contig_range() is proper to deal with bh like following patch.

Laura, can I have you name on Acked-by line?
Please let me represent my thanks.

Thanks for any feedback.

------------------------------- 8< ----------------------------------