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

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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>
This commit is contained in:
Vlastimil Babka 2016-01-14 15:19:23 -08:00 committed by Linus Torvalds
parent 6a15a37097
commit 48131e03ca
3 changed files with 53 additions and 56 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

42
fs/proc/task_mmu.c
View File

@ -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)

2
include/linux/shmem_fs.h
View File

@ -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)

65
mm/shmem.c
View File

@ -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 @@ unsigned long shmem_swap_usage(struct vm_area_struct *vma)
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.
*/