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Commit 48131e03 authored by Vlastimil Babka's avatar Vlastimil Babka Committed by Linus Torvalds
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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: default avatarVlastimil 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: default avatarMichal Hocko <mhocko@suse.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 6a15a370

fs/proc/task_mmu.c

+13 −29
Original line number Diff line number Diff line
@@ -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)

include/linux/shmem_fs.h

+2 −0
Original line number Diff line number Diff line
@@ -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)

mm/shmem.c

+38 −27
Original line number Diff line number Diff line
@@ -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.

 */