@@ -639,27 +639,24 @@ static bool scan_swap_map_try_ssd_cluster(struct swap_info_struct *si,
/*
* Other CPUs can use our cluster if they can't find a free cluster,
- * check if there is still free entry in the cluster
+ * check if the expected entry is still free. If not, drop it and
+ * reserve a new cluster.
*/
- max = min_t(unsigned long, si->max,
- ALIGN_DOWN(tmp, SWAPFILE_CLUSTER) + SWAPFILE_CLUSTER);
- if (tmp < max) {
- ci = lock_cluster(si, tmp);
- while (tmp < max) {
- if (!si->swap_map[tmp])
- break;
- tmp++;
- }
+ ci = lock_cluster(si, tmp);
+ if (si->swap_map[tmp]) {
unlock_cluster(ci);
- }
- if (tmp >= max) {
*cpu_next = SWAP_NEXT_NULL;
goto new_cluster;
}
+ unlock_cluster(ci);
+
*offset = tmp;
*scan_base = tmp;
+
+ max = ALIGN_DOWN(tmp, SWAPFILE_CLUSTER) + SWAPFILE_CLUSTER;
tmp += 1;
*cpu_next = tmp < max ? tmp : SWAP_NEXT_NULL;
+
return true;
}
When a CPU fails to reserve a cluster (due to free list exhaustion), we revert to the scanner to find a free entry somewhere in the swap file. This might cause an entry to be stolen from another CPU's reserved cluster. Upon noticing this, the CPU with the stolen entry would previously scan forward to the end of the cluster trying to find a free entry to use. If there were none, it would try to reserve a new pre-cpu cluster and allocate from that. This scanning behavior does not scale well to high-order allocations, which will be introduced in a future patch since would need to scan for a contiguous area that was naturally aligned. Given stealing is a rare occurrence, let's remove the scanning behavior from the ssd allocator and simply drop the cluster and try to allocate a new one. Given the purpose of the per-cpu cluster is to ensure a given task's pages are sequential on disk to aid readahead, allocating a new cluster at this point makes most sense. Furthermore, si->max will always be greater than or equal to the end of the last cluster because any partial cluster will never be put on the free cluster list. Therefore we can simplify this logic too. These changes make it simpler to generalize scan_swap_map_try_ssd_cluster() to handle any allocation order. Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> --- mm/swapfile.c | 21 +++++++++------------ 1 file changed, 9 insertions(+), 12 deletions(-)