Commit b50da6e9 authored by Zhaoyang Huang's avatar Zhaoyang Huang Committed by Linus Torvalds
Browse files

mm: fix a race on nr_swap_pages 

The scenario on which "Free swap = -4kB" happens in my system, which is caused
by several get_swap_pages racing with each other and show_swap_cache_info
happens simutaniously. No need to add a lock on get_swap_page_of_type as we
remove "Presub/PosAdd" here.

ProcessA			ProcessB			ProcessC
ngoals = 1			ngoals = 1
avail = nr_swap_pages(1)	avail = nr_swap_pages(1)
nr_swap_pages(1) -= ngoals
				nr_swap_pages(0) -= ngoals
								nr_swap_pages = -1

Link: https://lkml.kernel.org/r/1607050340-4535-1-git-send-email-zhaoyang.huang@unisoc.com


Signed-off-by: default avatarZhaoyang Huang <zhaoyang.huang@unisoc.com>
Acked-by: default avatarVlastimil Babka <vbabka@suse.cz>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 489e9fea

mm/swapfile.c

+6 −5
Original line number Diff line number Diff line
@@ -1042,16 +1042,18 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_size) 
	/* Only single cluster request supported */

	WARN_ON_ONCE(n_goal > 1 && size == SWAPFILE_CLUSTER);



	spin_lock(&swap_avail_lock);



	avail_pgs = atomic_long_read(&nr_swap_pages) / size;

	if (avail_pgs <= 0)

	if (avail_pgs <= 0) {

		spin_unlock(&swap_avail_lock);

		goto noswap;

	}



	n_goal = min3((long)n_goal, (long)SWAP_BATCH, avail_pgs);



	atomic_long_sub(n_goal * size, &nr_swap_pages);



	spin_lock(&swap_avail_lock);



start_over:

	node = numa_node_id();

	plist_for_each_entry_safe(si, next, &swap_avail_heads[node], avail_lists[node]) {
@@ -1125,14 +1127,13 @@ swp_entry_t get_swap_page_of_type(int type) 


	spin_lock(&si->lock);

	if (si->flags & SWP_WRITEOK) {

		atomic_long_dec(&nr_swap_pages);

		/* This is called for allocating swap entry, not cache */

		offset = scan_swap_map(si, 1);

		if (offset) {

			atomic_long_dec(&nr_swap_pages);

			spin_unlock(&si->lock);

			return swp_entry(type, offset);

		}

		atomic_long_inc(&nr_swap_pages);

	}

	spin_unlock(&si->lock);

fail: