[PATCH 0/23 v2] mm: Speedup page cache truncation

[Jan Kara]

Hello,

this is a second version of my patches to avoid clearing marks from xas_store()
and thus fix regression in page cache truncation.

Changes since v1
- rebased on 5.7-rc2
- drop xas_for_each_marked() fix as it was already merged
- reworked the whole series based on Matthew's feedback - we now create new
  function xas_store_noinit() and use it instead of changing xas_store()
  behavior. Note that for xas_store_range() and __xa_cmpxchg() I didn't bother
  to change names although they stop clearing marks as well. This is because
  there are only very few callers so it's easy to verify them, also chances of
  a clash with other patch introducing new callers are very small.

Original motivation:

Conversion of page cache to xarray (commit 69b6c1319b6 "mm: Convert truncate to
XArray" in particular) has regressed performance of page cache truncation
by about 10% (see my original report here [1]). This patch series aims at
improving the truncation to get some of that regression back.

The first patch fixes a long standing bug with xas_for_each_marked() that I've
uncovered when debugging my patches. The remaining patches then work towards
the ability to stop clearing marks in xas_store() which improves truncation
performance by about 6%.

The patches have passed radix_tree tests in tools/testing and also fstests runs
for ext4 & xfs.

								Honza

[1] https://lore.kernel.org/linux-mm/20190226165628.GB24711@xxxxxxxxxxxxxx