mm, proc: reduce cost of /proc/pid/smaps for unpopulated shmem mappings

Following the previous patch, further reduction of /proc/pid/smaps cost
is possible for private writable shmem mappings with unpopulated areas
where the page walk invokes the .pte_hole function.  We can use radix
tree iterator for each such area instead of calling find_get_entry() in
a loop.  This is possible at the extra maintenance cost of introducing
another shmem function shmem_partial_swap_usage().

To demonstrate the diference, I have measured this on a process that
creates a private writable 2GB mapping of a partially swapped out
/dev/shm/file (which cannot employ the optimizations from the prvious
patch) and doesn't populate it at all.  I time how long does it take to
cat /proc/pid/smaps of this process 100 times.

Before this patch:

real    0m3.831s
user    0m0.180s
sys     0m3.212s

After this patch:

real    0m1.176s
user    0m0.180s
sys     0m0.684s

The time is similar to the case where a radix tree iterator is employed
on the whole mapping.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jerome Marchand <jmarchan@redhat.com>
Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 53 insertions, 56 deletions

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 5830b2e129ed..8a03759bda38 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -488,42 +488,16 @@ static void smaps_account(struct mem_size_stats *mss, struct page *page,
 }
 
 #ifdef CONFIG_SHMEM
-static unsigned long smaps_shmem_swap(struct vm_area_struct *vma,
-                unsigned long addr)
-{
-        struct page *page;
-
-        page = find_get_entry(vma->vm_file->f_mapping,
-                                        linear_page_index(vma, addr));
-        if (!page)
-                return 0;
-
-        if (radix_tree_exceptional_entry(page))
-                return PAGE_SIZE;
-
-        page_cache_release(page);
-        return 0;
-
-}
-
 static int smaps_pte_hole(unsigned long addr, unsigned long end,
                 struct mm_walk *walk)
 {
         struct mem_size_stats *mss = walk->private;
 
-        while (addr < end) {
-                mss->swap += smaps_shmem_swap(walk->vma, addr);
-                addr += PAGE_SIZE;
-        }
+        mss->swap += shmem_partial_swap_usage(
+                        walk->vma->vm_file->f_mapping, addr, end);
 
         return 0;
 }
-#else
-static unsigned long smaps_shmem_swap(struct vm_area_struct *vma,
-                unsigned long addr)
-{
-        return 0;
-}
 #endif
 
 static void smaps_pte_entry(pte_t *pte, unsigned long addr,
@@ -555,7 +529,17 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
                         page = migration_entry_to_page(swpent);
         } else if (unlikely(IS_ENABLED(CONFIG_SHMEM) && mss->check_shmem_swap
                                                         && pte_none(*pte))) {
-                mss->swap += smaps_shmem_swap(vma, addr);
+                page = find_get_entry(vma->vm_file->f_mapping,
+                                                linear_page_index(vma, addr));
+                if (!page)
+                        return;
+
+                if (radix_tree_exceptional_entry(page))
+                        mss->swap += PAGE_SIZE;
+                else
+                        page_cache_release(page);
+
+                return;
         }
 
         if (!page)
diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index bd58be5e7a2a..a43f41cb3c43 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -61,6 +61,8 @@ extern void shmem_truncate_range(struct inode *inode, loff_t start, loff_t end);
 extern int shmem_unuse(swp_entry_t entry, struct page *page);
 
 extern unsigned long shmem_swap_usage(struct vm_area_struct *vma);
+extern unsigned long shmem_partial_swap_usage(struct address_space *mapping,
+                                                pgoff_t start, pgoff_t end);
 
 static inline struct page *shmem_read_mapping_page(
                                 struct address_space *mapping, pgoff_t index)
diff --git a/mm/shmem.c b/mm/shmem.c
index e978621de1ef..760d90cf2a41 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -361,41 +361,18 @@ static int shmem_free_swap(struct address_space *mapping,
 
 /*
  * Determine (in bytes) how many of the shmem object's pages mapped by the
- * given vma is swapped out.
+ * given offsets are swapped out.
  *
  * This is safe to call without i_mutex or mapping->tree_lock thanks to RCU,
  * as long as the inode doesn't go away and racy results are not a problem.
  */
-unsigned long shmem_swap_usage(struct vm_area_struct *vma)
+unsigned long shmem_partial_swap_usage(struct address_space *mapping,
+                                                pgoff_t start, pgoff_t end)
 {
-        struct inode *inode = file_inode(vma->vm_file);
-        struct shmem_inode_info *info = SHMEM_I(inode);
-        struct address_space *mapping = inode->i_mapping;
-        unsigned long swapped;
-        pgoff_t start, end;
         struct radix_tree_iter iter;
         void **slot;
         struct page *page;
-
-        /* Be careful as we don't hold info->lock */
-        swapped = READ_ONCE(info->swapped);
-
-        /*
-         * The easier cases are when the shmem object has nothing in swap, or
-         * the vma maps it whole. Then we can simply use the stats that we
-         * already track.
-         */
-        if (!swapped)
-                return 0;
-
-        if (!vma->vm_pgoff && vma->vm_end - vma->vm_start >= inode->i_size)
-                return swapped << PAGE_SHIFT;
-
-        swapped = 0;
-
-        /* Here comes the more involved part */
-        start = linear_page_index(vma, vma->vm_start);
-        end = linear_page_index(vma, vma->vm_end);
+        unsigned long swapped = 0;
 
         rcu_read_lock();
 
@@ -430,6 +407,40 @@ restart:
 }
 
 /*
+ * Determine (in bytes) how many of the shmem object's pages mapped by the
+ * given vma is swapped out.
+ *
+ * This is safe to call without i_mutex or mapping->tree_lock thanks to RCU,
+ * as long as the inode doesn't go away and racy results are not a problem.
+ */
+unsigned long shmem_swap_usage(struct vm_area_struct *vma)
+{
+        struct inode *inode = file_inode(vma->vm_file);
+        struct shmem_inode_info *info = SHMEM_I(inode);
+        struct address_space *mapping = inode->i_mapping;
+        unsigned long swapped;
+
+        /* Be careful as we don't hold info->lock */
+        swapped = READ_ONCE(info->swapped);
+
+        /*
+         * The easier cases are when the shmem object has nothing in swap, or
+         * the vma maps it whole. Then we can simply use the stats that we
+         * already track.
+         */
+        if (!swapped)
+                return 0;
+
+        if (!vma->vm_pgoff && vma->vm_end - vma->vm_start >= inode->i_size)
+                return swapped << PAGE_SHIFT;
+
+        /* Here comes the more involved part */
+        return shmem_partial_swap_usage(mapping,
+                        linear_page_index(vma, vma->vm_start),
+                        linear_page_index(vma, vma->vm_end));
+}
+
+/*
  * SysV IPC SHM_UNLOCK restore Unevictable pages to their evictable lists.
  */
 void shmem_unlock_mapping(struct address_space *mapping)