[PATCH v2 2/2] mm/ksm: prevent zero-page remapping during ksmd-triggered THP splits

["Nico Pache (Red Hat)" <nico.pache@linux.dev>]

Since commit b1f202060afe ("mm: remap unused subpages to shared zeropage
when splitting isolated thp"), splitting an anonymous THP remaps all
zero-filled subpages to the shared zeropage via TTU_USE_SHARED_ZEROPAGE.
This flag is set unconditionally for every anonymous folio split,
including splits triggered by KSM.

When KSM is enabled with THP=always, this causes two regressions:

1. use_zero_pages=1: KSM calls try_to_merge_one_page() which triggers
   split_huge_page(). The split remaps all 512 zero-filled subpages to
   the shared zeropage at once, freeing the entire 2MB THP when KSM only
   intended to process a single 4KB page. This bypasses KSM's
   pages_to_scan rate limiting, causing ~1GB to be freed almost
   instantly.

2. use_zero_pages=0: The same split side-effect occurs through the
   stable/unstable tree merge paths. Each pages_to_scan iteration
   triggers an expensive split_huge_page() that silently frees 2MB,
   while the scanner wastes cycles on tree searches for zero-filled
   pages that were already freed as a side-effect.

Introduce current_is_ksmd(), which uses kthread_func() to check whether
the current task is the KSM scanning thread, and use it in
try_to_map_unused_to_zeropage() to skip zero-page remapping when the
split is triggered by ksmd.

Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
Signed-off-by: Nico Pache (Red Hat) <nico.pache@linux.dev>
---
 include/linux/ksm.h | 6 ++++++
 mm/ksm.c            | 5 +++++
 mm/migrate.c        | 3 +++
 3 files changed, 14 insertions(+)

diff --git a/include/linux/ksm.h b/include/linux/ksm.h
index d39d0d5483a2..41c06a6abf60 100644
--- a/include/linux/ksm.h
+++ b/include/linux/ksm.h
@@ -17,6 +17,7 @@
 #ifdef CONFIG_KSM
 int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
 		unsigned long end, int advice, vm_flags_t *vm_flags);
+bool current_is_ksmd(void);
 vma_flags_t ksm_vma_flags(struct mm_struct *mm, const struct file *file,
 			  vma_flags_t vma_flags);
 int ksm_enable_merge_any(struct mm_struct *mm);
@@ -144,6 +145,11 @@ static inline int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
 	return 0;
 }
 
+static inline bool current_is_ksmd(void)
+{
+	return false;
+}
+
 static inline struct folio *ksm_might_need_to_copy(struct folio *folio,
 			struct vm_area_struct *vma, unsigned long addr)
 {
diff --git a/mm/ksm.c b/mm/ksm.c
index 910a1307ac9f..e3ab037b1ebe 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -2866,6 +2866,11 @@ static int ksm_scan_thread(void *nothing)
 	return 0;
 }
 
+bool current_is_ksmd(void)
+{
+	return kthread_func(current) == ksm_scan_thread;
+}
+
 static bool __ksm_should_add_vma(const struct file *file, vma_flags_t vma_flags)
 {
 	if (vma_flags_test(&vma_flags, VMA_MERGEABLE_BIT))
diff --git a/mm/migrate.c b/mm/migrate.c
index 7bdcdb57652f..c33ae3fa1043 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -305,6 +305,9 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
 	if (PageCompound(page) || PageHWPoison(page))
 		return false;
 
+	if (current_is_ksmd())
+		return false;
+
 	VM_BUG_ON_PAGE(!PageAnon(page), page);
 	VM_BUG_ON_PAGE(!PageLocked(page), page);
 	VM_BUG_ON_PAGE(pte_present(old_pte), page);

-- 
2.56.0