[PATCH 2/4] mm, procfs: account for shmem swap in /proc/pid/smaps

[Vlastimil Babka]

Currently, /proc/pid/smaps will always show "Swap: 0 kB" for shmem-backed
mappings, even if the mapped portion does contain pages that were swapped out.
This is because unlike private anonymous mappings, shmem does not change pte
to swap entry, but pte_none when swapping the page out. In the smaps page
walk, such page thus looks like it was never faulted in.

This patch changes smaps_pte_entry() to determine the swap status for such
pte_none entries for shmem mappings, similarly to how mincore_page() does it.
Swapped out pages are thus accounted for.

The accounting is arguably still not as precise as for private anonymous
mappings, since now we will count also pages that the process in question never
accessed, but only another process populated them and then let them become
swapped out. I believe it is still less confusing and subtle than not showing
any swap usage by shmem mappings at all. Also, swapped out pages only becomee a
performance issue for future accesses, and we cannot predict those for neither
kind of mapping.

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 Documentation/filesystems/proc.txt |  3 ++-
 fs/proc/task_mmu.c                 | 20 ++++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index d4f56ec..8b30543 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -437,7 +437,8 @@ indicates the amount of memory currently marked as referenced or accessed.
 a mapping associated with a file may contain anonymous pages: when MAP_PRIVATE
 and a page is modified, the file page is replaced by a private anonymous copy.
 "Swap" shows how much would-be-anonymous memory is also used, but out on
-swap.
+swap. For shmem mappings, "Swap" shows how much of the mapped portion of the
+underlying shmem object is on swap.
 
 "VmFlags" field deserves a separate description. This member represents the kernel
 flags associated with the particular virtual memory area in two letter encoded
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 956b75d..0410309 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -13,6 +13,7 @@
 #include <linux/swap.h>
 #include <linux/swapops.h>
 #include <linux/mmu_notifier.h>
+#include <linux/shmem_fs.h>
 
 #include <asm/elf.h>
 #include <asm/uaccess.h>
@@ -496,6 +497,25 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
 			mss->swap += PAGE_SIZE;
 		else if (is_migration_entry(swpent))
 			page = migration_entry_to_page(swpent);
+	} else if (IS_ENABLED(CONFIG_SHMEM) && IS_ENABLED(CONFIG_SWAP) &&
+					pte_none(*pte) && vma->vm_file) {
+		struct address_space *mapping =
+			file_inode(vma->vm_file)->i_mapping;
+
+		/*
+		 * shmem does not use swap pte's so we have to consult
+		 * the radix tree to account for swap
+		 */
+		if (shmem_mapping(mapping)) {
+			page = find_get_entry(mapping, pgoff);
+			if (page) {
+				if (radix_tree_exceptional_entry(page))
+					mss->swap += PAGE_SIZE;
+				else
+					page_cache_release(page);
+			}
+			page = NULL;
+		}
 	}
 
 	if (!page)
-- 
2.1.4

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