From a2d1c9ea7613f346a8ea215f0419af15b2912cbd Mon Sep 17 00:00:00 2001 From: "Kiryl Shutsemau (Meta)" Date: Wed, 14 Oct 2026 18:11:41 +0100 Subject: [PATCH] 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/20261014171145.3292736-9-kas@kernel.org Signed-off-by: Kiryl Shutsemau (Meta) Signed-off-by: Andrew Morton Acked-by: David Hildenbrand (Arm) Reviewed-by: Zi Yan Reviewed-by: Lance Yang Reviewed-by: Baolin Wang Assisted-by: LLM Cc: Lorenzo Stoakes Cc: Liam R. Howlett Cc: Nico Pache Cc: Ryan Roberts Cc: Dev Jain Cc: Barry Song Cc: Usama Arif Cc: Vlastimil Babka Cc: Jann Horn --- mm/khugepaged.c | 21 +++++++++++++-------- 1 file changed, 13 insertions(+), 8 deletions(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index a6cb9e92f7c9..7a6acb5ba863 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -2763,13 +2763,9 @@ 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); @@ -2800,8 +2796,16 @@ 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 && @@ -2813,6 +2817,7 @@ static enum scan_result collapse_single_pmd(unsigned long addr, triggered_wb = true; goto retry; } +put: fput(file); if (result == SCAN_PTE_MAPPED_HUGEPAGE) {