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Commit 31a56396 authored by Daisuke Nishimura's avatar Daisuke Nishimura Committed by Linus Torvalds
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mm: add_to_swap_cache() must not sleep 



After commit 355cfa73 ("mm: modify swap_map and add SWAP_HAS_CACHE flag"),
read_swap_cache_async() will busy-wait while a entry doesn't exist in swap
cache but it has SWAP_HAS_CACHE flag.

Such entries can exist on add/delete path of swap cache.  On add path,
add_to_swap_cache() is called soon after SWAP_HAS_CACHE flag is set, and
on delete path, swapcache_free() will be called (SWAP_HAS_CACHE flag is
cleared) soon after __delete_from_swap_cache() is called.  So, the
busy-wait works well in most cases.

But this mechanism can cause soft lockup if add_to_swap_cache() sleeps and
read_swap_cache_async() tries to swap-in the same entry on the same cpu.

This patch calls radix_tree_preload() before swapcache_prepare() and
divides add_to_swap_cache() into two part: radix_tree_preload() part and
radix_tree_insert() part(define it as __add_to_swap_cache()).

Signed-off-by: default avatarDaisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 8fbb398f

mm/swap_state.c

+46 −24
Original line number Diff line number Diff line
@@ -67,10 +67,10 @@ void show_swap_cache_info(void) 
}



/*

 * add_to_swap_cache resembles add_to_page_cache_locked on swapper_space,

 * __add_to_swap_cache resembles add_to_page_cache_locked on swapper_space,

 * but sets SwapCache flag and private instead of mapping and index.

 */

int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)

static int __add_to_swap_cache(struct page *page, swp_entry_t entry)

{

	int error;
@@ -78,28 +78,37 @@ int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask) 
	VM_BUG_ON(PageSwapCache(page));

	VM_BUG_ON(!PageSwapBacked(page));



	error = radix_tree_preload(gfp_mask);

	if (!error) {

	page_cache_get(page);

	SetPageSwapCache(page);

	set_page_private(page, entry.val);



	spin_lock_irq(&swapper_space.tree_lock);

		error = radix_tree_insert(&swapper_space.page_tree,

						entry.val, page);

	error = radix_tree_insert(&swapper_space.page_tree, entry.val, page);

	if (likely(!error)) {

		total_swapcache_pages++;

		__inc_zone_page_state(page, NR_FILE_PAGES);

		INC_CACHE_INFO(add_total);

	}

	spin_unlock_irq(&swapper_space.tree_lock);

		radix_tree_preload_end();



	if (unlikely(error)) {

		set_page_private(page, 0UL);

		ClearPageSwapCache(page);

		page_cache_release(page);

	}



	return error;

}





int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)

{

	int error;



	error = radix_tree_preload(gfp_mask);

	if (!error) {

		error = __add_to_swap_cache(page, entry);

		radix_tree_preload_end();

	}

	return error;

}
@@ -289,14 +298,25 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask, 
				break;		/* Out of memory */

		}



		/*

		 * call radix_tree_preload() while we can wait.

		 */

		err = radix_tree_preload(gfp_mask & GFP_KERNEL);

		if (err)

			break;



		/*

		 * Swap entry may have been freed since our caller observed it.

		 */

		err = swapcache_prepare(entry);

		if (err == -EEXIST) /* seems racy */

		if (err == -EEXIST) {	/* seems racy */

			radix_tree_preload_end();

			continue;

		if (err)           /* swp entry is obsolete ? */

		}

		if (err) {		/* swp entry is obsolete ? */

			radix_tree_preload_end();

			break;

		}



		/*

		 * Associate the page with swap entry in the swap cache.
@@ -308,8 +328,9 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask, 
		 */

		__set_page_locked(new_page);

		SetPageSwapBacked(new_page);

		err = add_to_swap_cache(new_page, entry, gfp_mask & GFP_KERNEL);

		err = __add_to_swap_cache(new_page, entry);

		if (likely(!err)) {

			radix_tree_preload_end();

			/*

			 * Initiate read into locked page and return.

			 */
@@ -317,6 +338,7 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask, 
			swap_readpage(new_page);

			return new_page;

		}

		radix_tree_preload_end();

		ClearPageSwapBacked(new_page);

		__clear_page_locked(new_page);

		swapcache_free(entry, NULL);