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Subject[PATCH v5 4/6] mm, proc: reduce cost of /proc/pid/smaps for unpopulated shmem mappings
Date
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 case where radix tree iterator is employed on the whole
mapping.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
fs/proc/task_mmu.c | 42 ++++++++++---------------------
include/linux/shmem_fs.h | 2 ++
mm/shmem.c | 65 ++++++++++++++++++++++++++++--------------------
3 files changed, 53 insertions(+), 56 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 491e675..a0338ec 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -503,42 +503,16 @@ 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,
@@ -570,7 +544,17 @@ 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
--- 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 bc0f676..8689a58 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 @@ unsigned long shmem_swap_usage(struct vm_area_struct *vma)
}

/*
+ * 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)
--
2.6.3


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