mm: folio_zero_user: cache neighbouring pages

mm: folio_zero_user: cache neighbouring pages

folio_zero_user() does straight zeroing without caring about temporal
locality for caches.

This replaced commit c6ddfb6c5890 ("mm, clear_huge_page: move order
algorithm into a separate function") where we cleared a page at a time
converging to the faulting page from the left and the right.

To retain limited temporal locality, split the clearing in three parts:
the faulting page and its immediate neighbourhood, and the regions on its
left and right.  We clear the local neighbourhood last to maximize chances
of it sticking around in the cache.

Performance
===

AMD Genoa (EPYC 9J14, cpus=2 sockets * 96 cores * 2 threads,
           memory=2.2 TB, L1d=16K/thread, L2=512K/thread, L3=2MB/thread)

vm-scalability/anon-w-seq-hugetlb: this workload runs with 384 processes
(one for each CPU) each zeroing anonymously mapped hugetlb memory which is
then accessed sequentially.  stime utime

  discontiguous-page      1739.93 ( +- 6.15% )  1016.61 ( +- 4.75% )
  contiguous-page         1853.70 ( +- 2.51% )  1187.13 ( +- 3.50% )
  batched-pages           1756.75 ( +- 2.98% )  1133.32 ( +- 4.89% )
  neighbourhood-last      1725.18 ( +- 4.59% )  1123.78 ( +- 7.38% )

Both stime and utime largely respond somewhat expectedly.  There is a fair
amount of run to run variation but the general trend is that the stime
drops and utime increases.  There are a few oddities, like contiguous-page
performing very differently from batched-pages.

As such this is likely an uncommon pattern where we saturate the memory
bandwidth (since all CPUs are running the test) and at the same time are
cache constrained because we access the entire region.

Kernel make (make -j 12 bzImage):

                              stime                  utime

  discontiguous-page      199.29 ( +- 0.63% )   1431.67 ( +- .04% )
  contiguous-page         193.76 ( +- 0.58% )   1433.60 ( +- .05% )
  batched-pages           193.92 ( +- 0.76% )   1431.04 ( +- .08% )
  neighbourhood-last      194.46 ( +- 0.68% )   1431.51 ( +- .06% )

For make the utime stays relatively flat with a fairly small (-2.4%)
improvement in the stime.

Link: https://lkml.kernel.org/r/20260107072009.1615991-9-ankur.a.arora@oracle.com
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
Reviewed-by: Raghavendra K T <raghavendra.kt@amd.com>
Tested-by: Raghavendra K T <raghavendra.kt@amd.com>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: "Borislav Petkov (AMD)" <bp@alien8.de>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Konrad Rzessutek Wilk <konrad.wilk@oracle.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Li Zhe <lizhe.67@bytedance.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>