[PATCH v2 0/2] mm/mprotect: micro-optimization work

[Pedro Falcato <pfalcato@suse.de>]

Micro-optimize the change_protection functionality and the
change_pte_range() routine. This set of functions works in an incredibly
tight loop, and even small inefficiencies are incredibly evident when spun
hundreds, thousands or hundreds of thousands of times.

There was an attempt to keep the batching functionality as much as possible,
which introduced some part of the slowness, but not all of it. Removing it
for !arm64 architectures would speed mprotect() up even further, but could
easily pessimize cases where large folios are mapped (which is not as rare
as it seems, particularly when it comes to the page cache these days).

The micro-benchmark used for the tests was [0] (usable using google/benchmark
and g++ -O2 -lbenchmark repro.cpp)

This resulted in the following (first entry is baseline):

---------------------------------------------------------
Benchmark               Time             CPU   Iterations
---------------------------------------------------------
mprotect_bench      85967 ns        85967 ns         6935
mprotect_bench      73374 ns        73373 ns         9602


After the patchset we can observe a 14% speedup in mprotect. Wonderful
for the elusive mprotect-based workloads!

Testing & more ideas welcome. I suspect there is plenty of improvement possible
but it would require more time than what I have on my hands right now. The
entire inlined function (which inlines into change_protection()) is gigantic
- I'm not surprised this is so finnicky.

Note: per my profiling, the next _big_ bottleneck here is modify_prot_start_ptes,
exactly on the xchg() done by x86. ptep_get_and_clear() is _expensive_. I don't think
there's a properly safe way to go about it since we do depend on the D bit
quite a lot. This might not be such an issue on other architectures.


[0]: https://gist.github.com/heatd/1450d273005aba91fa5744f44dfcd933
Link: https://lore.kernel.org/all/aY8-XuFZ7zCvXulB@luyang-thinkpadp1gen7.toromso.csb/

Cc: Vlastimil Babka <vbabka@kernel.org> 
Cc: Jann Horn <jannh@google.com> 
Cc: David Hildenbrand <david@kernel.org>
Cc: Dev Jain <dev.jain@arm.com> 
Cc: Luke Yang <luyang@redhat.com>
Cc: jhladky@redhat.com
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org

v2:
 - Addressed Sashiko's concerns
 - Picked up Lorenzo's R-b's (thank you!)
 - Squashed patch 1 and 4 into a single one (David)
 - Renamed the softleaf leaf function (David)
 - Dropped controversial noinlines & patch 3 (Lorenzo & David)

v1:
https://lore.kernel.org/linux-mm/20260319183108.1105090-1-pfalcato@suse.de/

Pedro Falcato (2):
  mm/mprotect: move softleaf code out of the main function
  mm/mprotect: special-case small folios when applying write permissions

 mm/mprotect.c | 146 ++++++++++++++++++++++++++++----------------------
 1 file changed, 81 insertions(+), 65 deletions(-)

-- 
2.53.0