Currently we remove pages from the radix tree one by one.  To speed up
page cache truncation, lock several pages at once and free them in one
go.  This allows us to batch radix tree operations in a more efficient
way and also save round-trips on mapping->tree_lock.  As a result we
gain about 20% speed improvement in page cache truncation.

Data from a simple benchmark timing 10000 truncates of 1024 pages (on
ext4 on ramdisk but the filesystem is barely visible in the profiles).
The range shows 1% and 95% percentiles of the measured times:

  4.14-rc2	4.14-rc2 + batched truncation
  248-256	209-219
  249-258	209-217
  248-255	211-239
  248-255	209-217
  247-256	210-218

[jack@suse.cz: convert delete_from_page_cache_batch() to pagevec]
  Link: http://lkml.kernel.org/r/20171018111648.13714-1-jack@suse.cz
[akpm@linux-foundation.org: move struct pagevec forward declaration to top-of-file]
Link: http://lkml.kernel.org/r/20171010151937.26984-8-jack@suse.czSigned-off-by: Jan Kara <jack@suse.cz>
Acked-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>