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Commit 46f65ec1 authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds
Browse files

tmpfs: convert shmem_unuse_inode to radix-swap 



Convert shmem_unuse_inode() to use a lockless gang lookup of the radix
tree, searching for matching swap.

This is somewhat slower than the old method: because of repeated radix
tree descents, because of copying entries up, but probably most because
the old method noted and skipped once a vector page was cleared of swap.
Perhaps we can devise a use of radix tree tagging to achieve that later.

shmem_add_to_page_cache() uses shmem_radix_tree_replace() to compensate
for the lockless lookup by checking that the expected entry is in place,
under lock.  It is not very satisfactory to be copying this much from
add_to_page_cache_locked(), but I think easier to sell than insisting
that every caller of add_to_page_cache*() go through the extras.

Signed-off-by: default avatarHugh Dickins <hughd@google.com>
Acked-by: default avatarRik van Riel <riel@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 7a5d0fbb

mm/shmem.c

+107 −26
Original line number Diff line number Diff line
@@ -261,6 +261,55 @@ static int shmem_radix_tree_replace(struct address_space *mapping, 
	return 0;

}



/*

 * Like add_to_page_cache_locked, but error if expected item has gone.

 */

static int shmem_add_to_page_cache(struct page *page,

				   struct address_space *mapping,

				   pgoff_t index, gfp_t gfp, void *expected)

{

	int error;



	VM_BUG_ON(!PageLocked(page));

	VM_BUG_ON(!PageSwapBacked(page));



	error = mem_cgroup_cache_charge(page, current->mm,

						gfp & GFP_RECLAIM_MASK);

	if (error)

		goto out;

	if (!expected)

		error = radix_tree_preload(gfp & GFP_RECLAIM_MASK);

	if (!error) {

		page_cache_get(page);

		page->mapping = mapping;

		page->index = index;



		spin_lock_irq(&mapping->tree_lock);

		if (!expected)

			error = radix_tree_insert(&mapping->page_tree,

							index, page);

		else

			error = shmem_radix_tree_replace(mapping, index,

							expected, page);

		if (!error) {

			mapping->nrpages++;

			__inc_zone_page_state(page, NR_FILE_PAGES);

			__inc_zone_page_state(page, NR_SHMEM);

			spin_unlock_irq(&mapping->tree_lock);

		} else {

			page->mapping = NULL;

			spin_unlock_irq(&mapping->tree_lock);

			page_cache_release(page);

		}

		if (!expected)

			radix_tree_preload_end();

	}

	if (error)

		mem_cgroup_uncharge_cache_page(page);

out:

	return error;

}



/*

 * Like find_get_pages, but collecting swap entries as well as pages.

 */
@@ -308,6 +357,42 @@ export: 
	return ret;

}



/*

 * Lockless lookup of swap entry in radix tree, avoiding refcount on pages.

 */

static pgoff_t shmem_find_swap(struct address_space *mapping, void *radswap)

{

	void  **slots[PAGEVEC_SIZE];

	pgoff_t indices[PAGEVEC_SIZE];

	unsigned int nr_found;



restart:

	nr_found = 1;

	indices[0] = -1;

	while (nr_found) {

		pgoff_t index = indices[nr_found - 1] + 1;

		unsigned int i;



		rcu_read_lock();

		nr_found = radix_tree_gang_lookup_slot(&mapping->page_tree,

					slots, indices, index, PAGEVEC_SIZE);

		for (i = 0; i < nr_found; i++) {

			void *item = radix_tree_deref_slot(slots[i]);

			if (radix_tree_deref_retry(item)) {

				rcu_read_unlock();

				goto restart;

			}

			if (item == radswap) {

				rcu_read_unlock();

				return indices[i];

			}

		}

		rcu_read_unlock();

		cond_resched();

	}

	return -1;

}



/*

 * Remove swap entry from radix tree, free the swap and its page cache.

 */
@@ -515,23 +600,21 @@ static void shmem_evict_inode(struct inode *inode) 
	end_writeback(inode);

}



/*

 * If swap found in inode, free it and move page from swapcache to filecache.

 */

static int shmem_unuse_inode(struct shmem_inode_info *info,

			     swp_entry_t swap, struct page *page)

{

	struct address_space *mapping = info->vfs_inode.i_mapping;

	void *radswap;

	pgoff_t index;

	int error;



	for (index = 0; index < SHMEM_NR_DIRECT; index++)

		if (shmem_get_swap(info, index).val == swap.val)

			goto found;

	radswap = swp_to_radix_entry(swap);

	index = shmem_find_swap(mapping, radswap);

	if (index == -1)

		return 0;

found:

	spin_lock(&info->lock);

	if (shmem_get_swap(info, index).val != swap.val) {

		spin_unlock(&info->lock);

		return 0;

	}



	/*

	 * Move _head_ to start search for next from here.
@@ -547,23 +630,30 @@ found: 
	 * but also to hold up shmem_evict_inode(): so inode cannot be freed

	 * beneath us (pagelock doesn't help until the page is in pagecache).

	 */

	error = add_to_page_cache_locked(page, mapping, index, GFP_NOWAIT);

	error = shmem_add_to_page_cache(page, mapping, index,

						GFP_NOWAIT, radswap);

	/* which does mem_cgroup_uncharge_cache_page on error */



	if (error != -ENOMEM) {

		/*

		 * Truncation and eviction use free_swap_and_cache(), which

		 * only does trylock page: if we raced, best clean up here.

		 */

		delete_from_swap_cache(page);

		set_page_dirty(page);

		shmem_put_swap(info, index, (swp_entry_t){0});

		if (!error) {

			spin_lock(&info->lock);

			info->swapped--;

			spin_unlock(&info->lock);

			swap_free(swap);

		}

		error = 1;	/* not an error, but entry was found */

	}

	spin_unlock(&info->lock);

	return error;

}



/*

 * shmem_unuse() search for an eventually swapped out shmem page.

 * Search through swapped inodes to find and replace swap by page.

 */

int shmem_unuse(swp_entry_t swap, struct page *page)

{
@@ -576,20 +666,12 @@ int shmem_unuse(swp_entry_t swap, struct page *page) 
	 * Charge page using GFP_KERNEL while we can wait, before taking

	 * the shmem_swaplist_mutex which might hold up shmem_writepage().

	 * Charged back to the user (not to caller) when swap account is used.

	 * add_to_page_cache() will be called with GFP_NOWAIT.

	 * shmem_add_to_page_cache() will be called with GFP_NOWAIT.

	 */

	error = mem_cgroup_cache_charge(page, current->mm, GFP_KERNEL);

	if (error)

		goto out;

	/*

	 * Try to preload while we can wait, to not make a habit of

	 * draining atomic reserves; but don't latch on to this cpu,

	 * it's okay if sometimes we get rescheduled after this.

	 */

	error = radix_tree_preload(GFP_KERNEL);

	if (error)

		goto uncharge;

	radix_tree_preload_end();

	/* No radix_tree_preload: swap entry keeps a place for page in tree */



	mutex_lock(&shmem_swaplist_mutex);

	list_for_each_safe(this, next, &shmem_swaplist) {
@@ -608,7 +690,6 @@ int shmem_unuse(swp_entry_t swap, struct page *page) 
	}

	mutex_unlock(&shmem_swaplist_mutex);



uncharge:

	if (!found)

		mem_cgroup_uncharge_cache_page(page);

	if (found < 0)