mm/collapse: call collapse_file() from collapse_single_pmd()

collapse_scan_file() reads the page cache to decide whether a table is
worth collapsing and, when it is, calls collapse_file() itself.  The
caller cannot get between the decision and the collapse.

Move the collapse_file() call up into collapse_single_pmd(), so the scan
stops at the decision.

Two things change with it.  The writeback retry re-runs collapse_file()
alone instead of rescanning first; collapse_file() repeats the scan's
checks under the page cache lock anyway.  And mm_khugepaged_scan_file
fires before the collapse, so for an accepted table its status reads
SCAN_SUCCEED; what the collapse made of the table is for
mm_khugepaged_collapse_file to report.

Preparation for splitting a collapse into a scan under mmap_lock and a
run without it.  The file scan has to stop where the anonymous one will.

Link: https://lore.kernel.org/20260928100630.21870-9-kirill@shutemov.name
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Assisted-by: LLM
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>

0 files changed, 0 insertions, 0 deletions