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Subject[PATCH] mm: swap: Use swapfiles in priority order
According to the swapon documentation

Swap pages are allocated from areas in priority order,
highest priority first. For areas with different priorities, a
higher-priority area is exhausted before using a lower-priority area.

A user reported that the reality is different. When multiple swap files
are enabled and a memory consumer started, the swap files are consumed in
pairs after the highest priority file is exhausted. Early in the lifetime
of the test, swapfile consumptions looks like

Filename Type Size Used Priority
/testswap1 file 100004 100004 8
/testswap2 file 100004 23764 7
/testswap3 file 100004 23764 6
/testswap4 file 100004 0 5
/testswap5 file 100004 0 4
/testswap6 file 100004 0 3
/testswap7 file 100004 0 2
/testswap8 file 100004 0 1

This patch fixes the swap_list search in get_swap_page to use the swap files
in the correct order. When applied the swap file consumptions looks like

Filename Type Size Used Priority
/testswap1 file 100004 100004 8
/testswap2 file 100004 100004 7
/testswap3 file 100004 29372 6
/testswap4 file 100004 0 5
/testswap5 file 100004 0 4
/testswap6 file 100004 0 3
/testswap7 file 100004 0 2
/testswap8 file 100004 0 1

Signed-off-by: Mel Gorman <mgorman@suse.de>
---
mm/swapfile.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 4a7f7e6..6d0ac2b 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -651,7 +651,7 @@ swp_entry_t get_swap_page(void)
goto noswap;
atomic_long_dec(&nr_swap_pages);

- for (type = swap_list.next; type >= 0 && wrapped < 2; type = next) {
+ for (type = swap_list.head; type >= 0 && wrapped < 2; type = next) {
hp_index = atomic_xchg(&highest_priority_index, -1);
/*
* highest_priority_index records current highest priority swap

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