Re: [RFC PATCH v2 02/11] mm: khugepaged: generalize collapse_file() for shmem mTHP support

[Baolin Wang]

On 6/12/26 6:16 PM, Nico Pache wrote:


On 6/10/26 4:29 AM, Baolin Wang wrote:
Generalize the order of the collapse_file() function to support future
shmem mTHP collapse.

No functional changes in this patch.

Signed-off-by: Baolin Wang <baolin.w...@linux.alibaba.com>
---
  mm/khugepaged.c | 27 +++++++++++++++------------
  1 file changed, 15 insertions(+), 12 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 631459172e19..4adc8c6de062 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2214,6 +2214,7 @@ static void retract_page_tables(struct address_space 
*mapping, pgoff_t pgoff)
   * @file: file that collapse on
   * @start: collapse start address
   * @cc: collapse context and scratchpad
+ * @order: folio order being collapsed to
   *
   * Basic scheme is simple, details are more complex:
   *  - allocate and lock a new huge page;
@@ -2232,15 +2233,17 @@ static void retract_page_tables(struct address_space 
*mapping, pgoff_t pgoff)
   *    + unlock and free huge page;
   */
  static enum scan_result collapse_file(struct mm_struct *mm, unsigned long 
addr,
-               struct file *file, pgoff_t start, struct collapse_control *cc)
+               struct file *file, pgoff_t start, struct collapse_control *cc,
+               int order)
  {
-       const unsigned int max_ptes_none = collapse_max_ptes_none(cc, NULL, 
HPAGE_PMD_ORDER);
+       const unsigned int max_ptes_none = collapse_max_ptes_none(cc, NULL, 
order);
        struct address_space *mapping = file->f_mapping;
+       const unsigned long nr_pages = 1UL << order;
        struct page *dst;
        struct folio *folio, *tmp, *new_folio;
-       pgoff_t index = 0, end = start + HPAGE_PMD_NR;
+       pgoff_t index = 0, end = start + nr_pages;
        LIST_HEAD(pagelist);
-       XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER);
+       XA_STATE_ORDER(xas, &mapping->i_pages, start, order);
        enum scan_result result = SCAN_SUCCEED;
        int nr_none = 0;
        bool is_shmem = shmem_file(file);
@@ -2252,9 +2255,9 @@ static enum scan_result collapse_file(struct mm_struct 
*mm, unsigned long addr,
         * mapping, the shmem check can be removed.
         */
        VM_WARN_ON_ONCE(!is_shmem && !mapping_pmd_folio_support(mapping));
-       VM_WARN_ON_ONCE(start & (HPAGE_PMD_NR - 1));
+       VM_WARN_ON_ONCE(start & (nr_pages - 1));
  
-	result = alloc_charge_folio(&new_folio, mm, cc, HPAGE_PMD_ORDER);
+       result = alloc_charge_folio(&new_folio, mm, cc, order);
        if (result != SCAN_SUCCEED)
                goto out;
  
@@ -2591,12 +2594,12 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
        }
  
  	if (is_shmem) {
-               lruvec_stat_mod_folio(new_folio, NR_SHMEM, HPAGE_PMD_NR);
+               lruvec_stat_mod_folio(new_folio, NR_SHMEM, nr_pages);
                lruvec_stat_mod_folio(new_folio, NR_SHMEM_THPS, HPAGE_PMD_NR);

Is this a accounting bug? (not your changes) but

lruvec_stat_mod_folio(new_folio, NR_SHMEM_THPS, HPAGE_PMD_NR);

If this stat is in THPs why are we iterating it by 512? shouldnt it just be +- 1

This is not a bug and this is intended behavior. Please refer to commit 
57b2847d3c1d ("mm: memcontrol: convert NR_SHMEM_THPS account to pages") 
for details.