Summary:
From other ones' investigation:
"sync_file_range() behavior highly depends on kernel version and filesystem.
xfs does neighbor page flushing outside of the specified ranges. For example, sync_file_range(fd, 8192, 16384) does not only trigger flushing page #3 to #4, but also flushing many more dirty pages (i.e. up to page#16)... Ranges of the sync_file_range() should be far enough from write() offset (at least 1MB)."
Test Plan: make all check
Reviewers: igor, rven, kradhakrishnan, yhchiang, IslamAbdelRahman, anthony
Reviewed By: anthony
Subscribers: yoshinorim, MarkCallaghan, sumeet, domas, dhruba, leveldb, ljin
Differential Revision: https://reviews.facebook.net/D15807
