On Wed 18-11-15 10:29:34, Vlastimil Babka wrote:

Following the previous patch, further reduction of /proc/pid/smaps cost is
possible for private writable shmem mappings with unpopulated areas where
the page walk invokes the .pte_hole function. We can use radix tree iterator
for each such area instead of calling find_get_entry() in a loop. This is
possible at the extra maintenance cost of introducing another shmem function
shmem_partial_swap_usage().

To demonstrate the diference, I have measured this on a process that creates a
private writable 2GB mapping of a partially swapped out /dev/shm/file (which
cannot employ the optimizations from the prvious patch) and doesn't populate it
at all. I time how long does it take to cat /proc/pid/smaps of this process 100
times.

Before this patch:

real    0m3.831s
user    0m0.180s
sys     0m3.212s

After this patch:

real    0m1.176s
user    0m0.180s
sys     0m0.684s

The time is similar to case where radix tree iterator is employed on the whole
mapping.

Signed-off-by: Vlastimil Babka <redacted>

Looks good as well.
Acked-by: Michal Hocko <mhocko@suse.com>
[...]
-- 
Michal Hocko
SUSE Labs

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