mm/collapse: separate scanning a PTE table from collapsing it

A collapse is two jobs.  One reads a PTE table under mmap_lock and decides
whether the range is worth collapsing.  The other allocates, isolates,
copies and flushes, and wants the lock given up first.

collapse_single_pmd() did both, so the boundary between them was somewhere
in the middle of a function.

Give each half its own function:

  - collapse_scan_pmd() scans one table.  The anonymous scan that used to
    carry that name keeps its body as collapse_scan_anon_pmd(), and
    collapse_scan_pmd() is now the entry that picks the anonymous or the
    file side.

  - collapse_run_pmd() does the collapse the scan asked for, and is
    handed what the scan returned.  SCAN_SUCCEED means there is
    something to collapse.  SCAN_PTE_MAPPED_HUGEPAGE means the page
    cache already holds the PMD folio and only the PTE table is left to
    retract.  Both are work for the run; anything else is why there is
    nothing to do.

collapse_single_pmd() is now the two of them with the mmap_lock drop in
between, so its callers see what they saw before.  collapse_control_init()
sets a control up before its first scan.

What the scan found and the run needs travels in collapse_control.  For
an anonymous table that is the orders and the referenced and swapped-out
counts, which mthp_collapse() and collapse_huge_page() now read from
there instead of taking as arguments.  For a file it is the file itself
and the offset in it: a file collapse works on the page cache and never
sees a VMA, so the scan takes the reference while it still has one and
the run gives it back.

The file scan moves under mmap_lock with the anonymous one, where before
the lock was given up first.  The lock is now held over the page cache
walk, an RCU walk over one table's worth of slots with no PTL, and taken
fewer times.  collapse_scan_mm_slot() ends its walk whenever the lock was
dropped, so a refused file table used to cost khugepaged an unlock, a
trip back through khugepaged_do_scan(), a relock and a VMA lookup.  Now
only a table that goes on to be collapsed does.

Link: https://lore.kernel.org/20260928100630.21870-10-kirill@shutemov.name
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Zi Yan <ziy@nvidia.com>
Assisted-by: LLM
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Nico Pache <nico.pache@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Jann Horn <jannh@google.com>

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