mm: add per-order mTHP swap-in fallback/fallback_charge counters

Currently, large folio swap-in is supported, but we lack a method to
analyze their success ratio.  Similar to anon_fault_fallback, we introduce
per-order mTHP swpin_fallback and swpin_fallback_charge counters for
calculating their success ratio.  The new counters are located at:

/sys/kernel/mm/transparent_hugepage/hugepages-<size>/stats/
	swpin_fallback
	swpin_fallback_charge

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Wenchao Hao <[email protected]>
Reviewed-by: Barry Song <[email protected]>
Reviewed-by: Lance Yang <[email protected]>
Cc: Baolin Wang <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Usama Arif <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: haowenchao

Documentation/admin-guide/mm/transhuge.rst

@@ -591,6 +591,16 @@ swpin
 	is incremented every time a huge page is swapped in from a non-zswap
 	swap device in one piece.
 
+swpin_fallback
+	is incremented if swapin fails to allocate or charge a huge page
+	and instead falls back to using huge pages with lower orders or
+	small pages.
+
+swpin_fallback_charge
+	is incremented if swapin fails to charge a huge page and instead
+	falls back to using  huge pages with lower orders or small pages
+	even though the allocation was successful.
+
 swpout
 	is incremented every time a huge page is swapped out to a non-zswap
 	swap device in one piece without splitting.

include/linux/huge_mm.h

@@ -121,6 +121,8 @@ enum mthp_stat_item {
 	MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE,
 	MTHP_STAT_ZSWPOUT,
 	MTHP_STAT_SWPIN,
+	MTHP_STAT_SWPIN_FALLBACK,
+	MTHP_STAT_SWPIN_FALLBACK_CHARGE,
 	MTHP_STAT_SWPOUT,
 	MTHP_STAT_SWPOUT_FALLBACK,
 	MTHP_STAT_SHMEM_ALLOC,

mm/huge_memory.c

@@ -617,6 +617,8 @@ DEFINE_MTHP_STAT_ATTR(anon_fault_fallback, MTHP_STAT_ANON_FAULT_FALLBACK);
 DEFINE_MTHP_STAT_ATTR(anon_fault_fallback_charge, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
 DEFINE_MTHP_STAT_ATTR(zswpout, MTHP_STAT_ZSWPOUT);
 DEFINE_MTHP_STAT_ATTR(swpin, MTHP_STAT_SWPIN);
+DEFINE_MTHP_STAT_ATTR(swpin_fallback, MTHP_STAT_SWPIN_FALLBACK);
+DEFINE_MTHP_STAT_ATTR(swpin_fallback_charge, MTHP_STAT_SWPIN_FALLBACK_CHARGE);
 DEFINE_MTHP_STAT_ATTR(swpout, MTHP_STAT_SWPOUT);
 DEFINE_MTHP_STAT_ATTR(swpout_fallback, MTHP_STAT_SWPOUT_FALLBACK);
 #ifdef CONFIG_SHMEM
@@ -637,6 +639,8 @@ static struct attribute *anon_stats_attrs[] = {
 #ifndef CONFIG_SHMEM
 	&zswpout_attr.attr,
 	&swpin_attr.attr,
+	&swpin_fallback_attr.attr,
+	&swpin_fallback_charge_attr.attr,
 	&swpout_attr.attr,
 	&swpout_fallback_attr.attr,
 #endif
@@ -669,6 +673,8 @@ static struct attribute *any_stats_attrs[] = {
 #ifdef CONFIG_SHMEM
 	&zswpout_attr.attr,
 	&swpin_attr.attr,
+	&swpin_fallback_attr.attr,
+	&swpin_fallback_charge_attr.attr,
 	&swpout_attr.attr,
 	&swpout_fallback_attr.attr,
 #endif

mm/memory.c

@@ -4258,8 +4258,10 @@ static struct folio *alloc_swap_folio(struct vm_fault *vmf)
 			if (!mem_cgroup_swapin_charge_folio(folio, vma->vm_mm,
 							    gfp, entry))
 				return folio;
+			count_mthp_stat(order, MTHP_STAT_SWPIN_FALLBACK_CHARGE);
 			folio_put(folio);
 		}
+		count_mthp_stat(order, MTHP_STAT_SWPIN_FALLBACK);
 		order = next_order(&orders, order);
 	}