[PATCH v5 8/9] mm/collapse: call collapse_file() from collapse_single_pmd()

["Kiryl Shutsemau (Meta)" <kas@kernel.org>]

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.

Assisted-by: LLM
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 mm/khugepaged.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 60ec7e80d554..89a4c3f5a91c 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2737,13 +2737,9 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
 	else
 		cc->progress += HPAGE_PMD_NR;
 
-	if (result == SCAN_SUCCEED) {
-		if (present < HPAGE_PMD_NR - max_ptes_none) {
-			result = SCAN_EXCEED_NONE_PTE;
-			count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
-		} else {
-			result = collapse_file(mm, addr, file, start, cc);
-		}
+	if (result == SCAN_SUCCEED && present < HPAGE_PMD_NR - max_ptes_none) {
+		result = SCAN_EXCEED_NONE_PTE;
+		count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
 	}
 
 	trace_mm_khugepaged_scan_file(mm, failed_pfn, file, present, swap, result);
@@ -2774,8 +2770,16 @@ static enum scan_result collapse_single_pmd(unsigned long addr,
 
 	mmap_read_unlock(mm);
 	*lock_dropped = true;
-retry:
+
+	/*
+	 * SCAN_PTE_MAPPED_HUGEPAGE is work too: the page cache already holds
+	 * the PMD folio, and only the PTE table is left to retract.
+	 */
 	result = collapse_scan_file(mm, addr, file, pgoff, cc);
+	if (result != SCAN_SUCCEED)
+		goto put;
+retry:
+	result = collapse_file(mm, addr, file, pgoff, cc);
 
 	/* Dirty pages are worth a writeback and one more try, if asked for */
 	if (cc->policy.file_writeback_dirty && result == SCAN_PAGE_DIRTY_OR_WRITEBACK &&
@@ -2787,6 +2791,7 @@ static enum scan_result collapse_single_pmd(unsigned long addr,
 		triggered_wb = true;
 		goto retry;
 	}
+put:
 	fput(file);
 
 	if (result == SCAN_PTE_MAPPED_HUGEPAGE) {
-- 
2.54.0