[PATCH v2 3/3] mm/damon/paddr: support hugetlb folios in access monitoring

[Krishna Iyer <kiyer@crusoe.ai>]

DAMON's physical address space monitoring is blind to hugetlb-backed
memory.  Every access check starts at damon_get_folio(), which rejects
folios that are not on the LRU lists.  Hugetlb folios are managed
outside of the LRU by design, so every sampling attempt on
hugetlb-backed memory silently fails and the pages are reported as
never accessed.

This is a significant blind spot on virtualization hosts.  Cloud
hypervisor hosts commonly back guest memory with 1 GiB hugetlbfs pages,
covering the vast majority of the machine's memory.  On such hosts,
modules like DAMON_STAT observe only the host-side remainder (page
cache, daemons) and report all guest working sets as permanently
idle, defeating the purpose of host-level access monitoring.  In
testing on a 1 TiB host, an hour of 4-thread random access over 842 GiB
inside a guest was statistically indistinguishable from an idle host,
while a 40x smaller host-side workload produced a quantitatively
correct response.

Add damon_get_monitor_folio(), which additionally accepts hugetlb
folios, and use it in the two paddr access monitoring primitives,
damon_pa_mkold() and damon_pa_young().  With the previous commit
teaching the folio-granular rmap walkers to age huge PTEs and to call
the mmu notifiers spanning the whole huge page, this makes guest
accesses visible through secondary MMU (e.g. KVM/EPT) young bits.

Free hugetlb pool folios have a zero refcount, so folio_try_get()
naturally keeps rejecting them.

The DAMOS action appliers (damon_pa_pageout(),
damon_pa_mark_accessed_or_deactivate(), damon_pa_migrate(),
damon_pa_stat()) keep using damon_get_folio(): reclaim, LRU
manipulation and migration cannot act on hugetlb folios, so their
behavior is unchanged.

Note that the access check granularity for hugetlb-backed memory is the
huge page size: one touched byte reports the whole (up to 1 GiB) page
as accessed.  Also, DAMON now consumes secondary MMU young bits that
KVM's own aging uses; at DAMON's sampling rate (one page per region per
sampling interval) the interference is negligible.

Assisted-by: Claude:claude-fable-5
Signed-off-by: Krishna Iyer <kiyer@crusoe.ai>
---
 mm/damon/ops-common.c | 31 +++++++++++++++++++++++++++----
 mm/damon/ops-common.h |  1 +
 mm/damon/paddr.c      |  4 ++--
 3 files changed, 30 insertions(+), 6 deletions(-)

diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c
index 373b25db5950..80207b56c6fd 100644
--- a/mm/damon/ops-common.c
+++ b/mm/damon/ops-common.c
@@ -15,14 +15,20 @@
 #include "../internal.h"
 #include "ops-common.h"
 
+static bool damon_folio_acceptable(struct folio *folio, bool monitor)
+{
+	return folio_test_lru(folio) ||
+		(monitor && folio_test_hugetlb(folio));
+}
+
 /*
- * Get an online page for a pfn if it's in the LRU list.  Otherwise, returns
- * NULL.
+ * Get an online page for a pfn if it's in the LRU list, or a hugetlb folio if
+ * @monitor is set.  Otherwise, returns NULL.
  *
  * The body of this function is stolen from the 'page_idle_get_folio()'.  We
  * steal rather than reuse it because the code is quite simple.
  */
-struct folio *damon_get_folio(unsigned long pfn)
+static struct folio *__damon_get_folio(unsigned long pfn, bool monitor)
 {
 	struct page *page = pfn_to_online_page(pfn);
 	struct folio *folio;
@@ -33,13 +39,30 @@ struct folio *damon_get_folio(unsigned long pfn)
 	folio = page_folio(page);
 	if (!folio_try_get(folio))
 		return NULL;
-	if (unlikely(page_folio(page) != folio) || !folio_test_lru(folio)) {
+	if (unlikely(page_folio(page) != folio) ||
+			!damon_folio_acceptable(folio, monitor)) {
 		folio_put(folio);
 		folio = NULL;
 	}
 	return folio;
 }
 
+struct folio *damon_get_folio(unsigned long pfn)
+{
+	return __damon_get_folio(pfn, false);
+}
+
+/*
+ * Same to damon_get_folio(), but also accepts hugetlb folios, which are
+ * managed outside of the LRU lists.  Aimed to be used by access monitoring
+ * primitives.  DAMOS actions that assume LRU-managed folios should keep using
+ * damon_get_folio().
+ */
+struct folio *damon_get_monitor_folio(unsigned long pfn)
+{
+	return __damon_get_folio(pfn, true);
+}
+
 void damon_ptep_mkold(pte_t *pte, struct vm_area_struct *vma, unsigned long addr)
 {
 	pte_t pteval = ptep_get(pte);
diff --git a/mm/damon/ops-common.h b/mm/damon/ops-common.h
index f7811c9c7a02..172f0f17c4a8 100644
--- a/mm/damon/ops-common.h
+++ b/mm/damon/ops-common.h
@@ -6,6 +6,7 @@
 #include <linux/damon.h>
 
 struct folio *damon_get_folio(unsigned long pfn);
+struct folio *damon_get_monitor_folio(unsigned long pfn);
 
 void damon_ptep_mkold(pte_t *pte, struct vm_area_struct *vma, unsigned long addr);
 void damon_pmdp_mkold(pmd_t *pmd, struct vm_area_struct *vma, unsigned long addr);
diff --git a/mm/damon/paddr.c b/mm/damon/paddr.c
index 5c6c3a597fd0..ed7b7f31291a 100644
--- a/mm/damon/paddr.c
+++ b/mm/damon/paddr.c
@@ -37,7 +37,7 @@ static unsigned long damon_pa_core_addr(
 
 static void damon_pa_mkold(phys_addr_t paddr)
 {
-	struct folio *folio = damon_get_folio(PHYS_PFN(paddr));
+	struct folio *folio = damon_get_monitor_folio(PHYS_PFN(paddr));
 
 	if (!folio)
 		return;
@@ -67,7 +67,7 @@ static void damon_pa_prepare_access_checks(struct damon_ctx *ctx)
 
 static bool damon_pa_young(phys_addr_t paddr)
 {
-	struct folio *folio = damon_get_folio(PHYS_PFN(paddr));
+	struct folio *folio = damon_get_monitor_folio(PHYS_PFN(paddr));
 	bool accessed;
 
 	if (!folio)
-- 
2.54.0