mm: khugepaged: refine scan progress number

Currently, each scan always increases "progress" by HPAGE_PMD_NR,
even if only scanning a single PTE/PMD entry.

- When only scanning a sigle PTE entry, let me provide a detailed
example:

static int hpage_collapse_scan_pmd()
{
for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR;
_pte++, addr += PAGE_SIZE) {
pte_t pteval = ptep_get(_pte);
...
if (pte_uffd_wp(pteval)) { <-- first scan hit
result = SCAN_PTE_UFFD_WP;
goto out_unmap;
}
}
}

During the first scan, if pte_uffd_wp(pteval) is true, the loop exits
directly. In practice, only one PTE is scanned before termination. Here,
"progress += 1" reflects the actual number of PTEs scanned, but previously
"progress += HPAGE_PMD_NR" always.

- When the memory has been collapsed to PMD, let me provide a detailed
example:

The following data is traced by bpftrace on a desktop system. After the
system has been left idle for 10 minutes upon booting, a lot of
SCAN_PMD_MAPPED or SCAN_NO_PTE_TABLE are observed during a full scan by
khugepaged.

From trace_mm_khugepaged_scan_pmd and trace_mm_khugepaged_scan_file, the
following statuses were observed, with frequency mentioned next to them:

SCAN_SUCCEED : 1
SCAN_EXCEED_SHARED_PTE: 2
SCAN_PMD_MAPPED : 142
SCAN_NO_PTE_TABLE : 178
total progress size : 674 MB
Total time : 419 seconds, include khugepaged_scan_sleep_millisecs

The khugepaged_scan list save all task that support collapse into
hugepage, as long as the task is not destroyed, khugepaged will not remove
it from the khugepaged_scan list. This exist a phenomenon where task has
already collapsed all memory regions into hugepage, but khugepaged
continues to scan it, which wastes CPU time and invalid, and due to
khugepaged_scan_sleep_millisecs (default 10s) causes a long wait for
scanning a large number of invalid task, so scanning really valid task is
later.

After applying this patch, when the memory is either SCAN_PMD_MAPPED or
SCAN_NO_PTE_TABLE, just skip it, as follow:

SCAN_EXCEED_SHARED_PTE: 2
SCAN_PMD_MAPPED : 147
SCAN_NO_PTE_TABLE : 173
total progress size : 45 MB
Total time : 20 seconds

SCAN_PTE_MAPPED_HUGEPAGE is the same, for detailed data, refer to
https://lore.kernel.org/linux-mm/4qdu7owpmxfh3ugsue775fxarw5g2gcggbxdf5psj75nnu7z2u@cv2uu2yocaxq

Link: https://lkml.kernel.org/r/20260221093918.1456187-3-vernon2gm@gmail.com
Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand (arm) <david@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam Howlett <Liam.Howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>