mm/mprotect: move softleaf code out of the main function

mm/mprotect: move softleaf code out of the main function

Patch series "mm/mprotect: micro-optimization work", v3.

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      70684 ns        70684 ns         9887

After the patchset we can observe an ~18% 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.

Luke Yang reported [1]:

: On average, we see improvements ranging from a minimum of 5% to a
: maximum of 55%, with most improvements showing around a 25% speed up in
: the libmicro/mprot_tw4m micro benchmark.

This patch (of 2):

Move softleaf change_pte_range code into a separate function.  This makes
the change_pte_range() function a good bit smaller, and lessens cognitive
load when reading through the function.

Link: https://lore.kernel.org/20260402141628.3367596-1-pfalcato@suse.de
Link: https://lore.kernel.org/20260402141628.3367596-2-pfalcato@suse.de
Link: https://lore.kernel.org/all/aY8-XuFZ7zCvXulB@luyang-thinkpadp1gen7.toromso.csb/
Link: https://gist.github.com/heatd/1450d273005aba91fa5744f44dfcd933
Link: https://lore.kernel.org/CAL2CeBxT4jtJ+LxYb6=BNxNMGinpgD_HYH5gGxOP-45Q2OncqQ@mail.gmail.com
Signed-off-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Tested-by: Luke Yang <luyang@redhat.com>
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jiri Hladky <jhladky@redhat.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>