On Thu, Sep 11, 2025 at 9:43 PM Lorenzo Stoakes
[off-list ref] wrote:

On Wed, Sep 10, 2025 at 10:44:38AM +0800, Yafang Shao wrote:

quoted

Since a task with MMF_DISABLE_THP_COMPLETELY cannot use THP, remove it from
the khugepaged_mm_slot to stop khugepaged from processing it.

After this change, the following semantic relationship always holds:

  MMF_VM_HUGEPAGE is set     == task is in khugepaged mm_slot
  MMF_VM_HUGEPAGE is not set == task is not in khugepaged mm_slot

Signed-off-by: Yafang Shao <redacted>
Cc: Lance Yang <redacted>

(Obviously on basis of fixing issue bot reported).

quoted

---
 include/linux/khugepaged.h |  1 +
 kernel/sys.c               |  6 ++++++
 mm/khugepaged.c            | 19 +++++++++----------
 3 files changed, 16 insertions(+), 10 deletions(-)

diff --git a/include/linux/khugepaged.h b/include/linux/khugepaged.h
index eb1946a70cff..6cb9107f1006 100644
--- a/include/linux/khugepaged.h
+++ b/include/linux/khugepaged.h

@@ -19,6 +19,7 @@ extern void khugepaged_min_free_kbytes_update(void);
 extern bool current_is_khugepaged(void);
 extern int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
                                 bool install_pmd);
+bool hugepage_pmd_enabled(void);

Need to provide a !CONFIG_TRANSPARENT_HUGEPAGE version, or to not invoke
this in a context where CONFIG_TRANSPARENT_HUGEPAGE is specified.

quoted

 static inline void khugepaged_fork(struct mm_struct *mm, struct mm_struct *oldmm)
 {

diff --git a/kernel/sys.c b/kernel/sys.c
index a46d9b75880b..a1c1e8007f2d 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c

@@ -8,6 +8,7 @@
 #include <linux/export.h>
 #include <linux/mm.h>
 #include <linux/mm_inline.h>
+#include <linux/khugepaged.h>
 #include <linux/utsname.h>
 #include <linux/mman.h>
 #include <linux/reboot.h>

@@ -2493,6 +2494,11 @@ static int prctl_set_thp_disable(bool thp_disable, unsigned long flags,
              mm_flags_clear(MMF_DISABLE_THP_COMPLETELY, mm);
              mm_flags_clear(MMF_DISABLE_THP_EXCEPT_ADVISED, mm);
      }
+
+     if (!mm_flags_test(MMF_DISABLE_THP_COMPLETELY, mm) &&
+         !mm_flags_test(MMF_VM_HUGEPAGE, mm) &&
+         hugepage_pmd_enabled())
+             __khugepaged_enter(mm);

Let's refactor this so it's not open-coded.

We can have:

void khugepaged_enter_mm(struct mm_struct *mm)
{
        if (mm_flags_test(MMF_DISABLE_THP_COMPLETELY, mm))
                return;
        if (mm_flags_test(MMF_VM_HUGEPAGE, mm))
                return;
        if (!hugepage_pmd_enabled())
                return;

        __khugepaged_enter(mm);
}

void khugepaged_enter_vma(struct vm_area_struct *vma,
                          vm_flags_t vm_flags)
{
        if (!thp_vma_allowable_order(vma, vm_flags, TVA_KHUGEPAGED, PMD_ORDER))
                return;

        khugepaged_enter_mm(vma->vm_mm);
}

Then just invoke khugepaged_enter_mm() here.

That's better.

quoted

      mmap_write_unlock(current->mm);
      return 0;
 }

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 4ec324a4c1fe..88ac482fb3a0 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c

@@ -413,7 +413,7 @@ static inline int hpage_collapse_test_exit_or_disable(struct mm_struct *mm)
              mm_flags_test(MMF_DISABLE_THP_COMPLETELY, mm);
 }

-static bool hugepage_pmd_enabled(void)
+bool hugepage_pmd_enabled(void)
 {
      /*
       * We cover the anon, shmem and the file-backed case here; file-backed

@@ -445,6 +445,7 @@ void __khugepaged_enter(struct mm_struct *mm)

      /* __khugepaged_exit() must not run from under us */
      VM_BUG_ON_MM(hpage_collapse_test_exit(mm), mm);
+     WARN_ON_ONCE(mm_flags_test(MMF_DISABLE_THP_COMPLETELY, mm));

Not sure why this needs to be a naked WARN_ON_ONCE()? Seems that'd be a
programmatic eror, so VM_WARN_ON_ONCE() more appropriate?

ack

Can also change the VM_BUG_ON_MM() to VM_WARN_ON_ONCE_MM() while we're here.

ack

quoted

      if (unlikely(mm_flags_test_and_set(MMF_VM_HUGEPAGE, mm)))
              return;

@@ -472,7 +473,8 @@ void __khugepaged_enter(struct mm_struct *mm)
 void khugepaged_enter_vma(struct vm_area_struct *vma,
                        vm_flags_t vm_flags)
 {
-     if (!mm_flags_test(MMF_VM_HUGEPAGE, vma->vm_mm) &&
+     if (!mm_flags_test(MMF_DISABLE_THP_COMPLETELY, vma->vm_mm) &&
+         !mm_flags_test(MMF_VM_HUGEPAGE, vma->vm_mm) &&
          hugepage_pmd_enabled()) {
              if (thp_vma_allowable_order(vma, vm_flags, TVA_KHUGEPAGED, PMD_ORDER))
                      __khugepaged_enter(vma->vm_mm);

See above, we can refactor this.

ack

quoted

@@ -1451,16 +1453,13 @@ static void collect_mm_slot(struct khugepaged_mm_slot *mm_slot)

      lockdep_assert_held(&khugepaged_mm_lock);

-     if (hpage_collapse_test_exit(mm)) {
+     if (hpage_collapse_test_exit_or_disable(mm)) {
              /* free mm_slot */
              hash_del(&slot->hash);
              list_del(&slot->mm_node);

-             /*
-              * Not strictly needed because the mm exited already.
-              *
-              * mm_flags_clear(MMF_VM_HUGEPAGE, mm);
-              */
+             /* If the mm is disabled, this flag must be cleared. */
+             mm_flags_clear(MMF_VM_HUGEPAGE, mm);

              /* khugepaged_mm_lock actually not necessary for the below */
              mm_slot_free(mm_slot_cache, mm_slot);

@@ -2507,9 +2506,9 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
      VM_BUG_ON(khugepaged_scan.mm_slot != mm_slot);
      /*
       * Release the current mm_slot if this mm is about to die, or
-      * if we scanned all vmas of this mm.
+      * if we scanned all vmas of this mm, or if this mm is disabled.
       */
-     if (hpage_collapse_test_exit(mm) || !vma) {
+     if (hpage_collapse_test_exit_or_disable(mm) || !vma) {
              /*
               * Make sure that if mm_users is reaching zero while
               * khugepaged runs here, khugepaged_exit will find

Seems reasonable, but makes me wonder if we actually always want to invoke
hpage_collapse_test_exit_or_disable()?

I guess the VM_BUG_ON() (though it should be a VM_WARN_ON_ONCE()) in
__khugepaged_enter() is a legit use, but the only other case is
retract_page_tables().

I wonder if we should change this also? Seems reasonable to.

Right, we can change it also. Thanks for your suggestion.


-- 
Regards
Yafang