mm, shmem: add vmstat for hugepage fallback

The existing thp_fault_fallback indicates when thp attempts to allocate a
hugepage but fails, or if the hugepage cannot be charged to the mem cgroup
hierarchy.

Extend this to shmem as well.  Adds a new thp_file_fallback to complement
thp_file_alloc that gets incremented when a hugepage is attempted to be
allocated but fails, or if it cannot be charged to the mem cgroup
hierarchy.

Additionally, remove the check for CONFIG_TRANSPARENT_HUGE_PAGECACHE from
shmem_alloc_hugepage() since it is only called with this configuration
option.
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Yang Shi <yang.shi@linux.alibaba.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Jeremy Cline <jcline@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Link: http://lkml.kernel.org/r/alpine.DEB.2.21.2003061421240.7412@chino.kir.corp.google.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/admin-guide/mm/transhuge.rst

@@ -319,6 +319,10 @@ thp_file_alloc
 	is incremented every time a file huge page is successfully
 	allocated.
 
+thp_file_fallback
+	is incremented if a file huge page is attempted to be allocated
+	but fails and instead falls back to using small pages.
+
 thp_file_mapped
 	is incremented every time a file huge page is mapped into
 	user address space.

include/linux/vm_event_item.h

@@ -76,6 +76,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
 		THP_COLLAPSE_ALLOC,
 		THP_COLLAPSE_ALLOC_FAILED,
 		THP_FILE_ALLOC,
+		THP_FILE_FALLBACK,
 		THP_FILE_MAPPED,
 		THP_SPLIT_PAGE,
 		THP_SPLIT_PAGE_FAILED,
@@ -115,6 +116,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
 
 #ifndef CONFIG_TRANSPARENT_HUGEPAGE
 #define THP_FILE_ALLOC ({ BUILD_BUG(); 0; })
+#define THP_FILE_FALLBACK ({ BUILD_BUG(); 0; })
 #define THP_FILE_MAPPED ({ BUILD_BUG(); 0; })
 #endif
 

mm/shmem.c

@@ -1472,9 +1472,6 @@ static struct page *shmem_alloc_hugepage(gfp_t gfp,
 	pgoff_t hindex;
 	struct page *page;
 
-	if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE))
-		return NULL;
-
 	hindex = round_down(index, HPAGE_PMD_NR);
 	if (xa_find(&mapping->i_pages, &hindex, hindex + HPAGE_PMD_NR - 1,
 								XA_PRESENT))
@@ -1486,6 +1483,8 @@ static struct page *shmem_alloc_hugepage(gfp_t gfp,
 	shmem_pseudo_vma_destroy(&pvma);
 	if (page)
 		prep_transhuge_page(page);
+	else
+		count_vm_event(THP_FILE_FALLBACK);
 	return page;
 }
 
@@ -1871,8 +1870,11 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
 
 	error = mem_cgroup_try_charge_delay(page, charge_mm, gfp, &memcg,
 					    PageTransHuge(page));
-	if (error)
+	if (error) {
+		if (PageTransHuge(page))
+			count_vm_event(THP_FILE_FALLBACK);
 		goto unacct;
+	}
 	error = shmem_add_to_page_cache(page, mapping, hindex,
 					NULL, gfp & GFP_RECLAIM_MASK);
 	if (error) {

mm/vmstat.c

@@ -1259,6 +1259,7 @@ const char * const vmstat_text[] = {
 	"thp_collapse_alloc",
 	"thp_collapse_alloc_failed",
 	"thp_file_alloc",
+	"thp_file_fallback",
 	"thp_file_mapped",
 	"thp_split_page",
 	"thp_split_page_failed",