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>

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 5830b2e..8a03759 100644 (file)
--- 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 bd58be5..a43f41c 100644 (file)
--- 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 e978621..760d90c 100644 (file)
--- 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();
 
@@ -429,6 +406,40 @@ restart:
        return swapped << PAGE_SHIFT;
 }
 
+/*
+ * 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.
  */