Re: [PATCH] mm/swapfile.c: select the swap device with default priority round robin

[Baoquan He <bhe@redhat.com>]

On 09/24/25 at 05:17pm, Baoquan He wrote:
> Currently, on system with multiple swap devices, swap allocation will
> select one swap device according to priority. The swap device with the
> highest priority will be chosen to allocate firstly.
> 
> People can specify a priority from 0 to 32767 when swapon a swap device,
> or the system will set it from -2 then downwards by default. Meanwhile,
> on NUMA system, the swap device with node_id will be considered first
> on that NUMA node of the node_id.
> 
> In the current code, an array of plist, swap_avail_heads[nid], is used
> to organize swap devices on each NUMA node. For each NUMA node, there
> is a plist organizing all swap devices. The 'prio' value in the plist
> is the negated value of the device's priority due to plist being sorted
> from low to high. The swap device owning one node_id will be promoted to
> the front position on that NUMA node, then other swap devices are put in
> order of their default priority.
> 
> E.g I got a system with 8 NUMA nodes, and I setup 4 zram partition as
> swap devices.
> 
> Current behaviour:
> their priorities will be(note that -1 is skipped):
> NAME       TYPE      SIZE USED PRIO
> /dev/zram0 partition  16G   0B   -2
> /dev/zram1 partition  16G   0B   -3
> /dev/zram2 partition  16G   0B   -4
> /dev/zram3 partition  16G   0B   -5
> 
> And their positions in the 8 swap_avail_lists[nid] will be:
> swap_avail_lists[0]: /* node 0's available swap device list */
> zram0   -> zram1   -> zram2   -> zram3
> prio:1     prio:3     prio:4     prio:5
> swap_avali_lists[1]: /* node 1's available swap device list */
> zram1   -> zram0   -> zram2   -> zram3
> prio:1     prio:2     prio:4     prio:5
> swap_avail_lists[2]: /* node 2's available swap device list */
> zram2   -> zram0   -> zram1   -> zram3
> prio:1     prio:2     prio:3     prio:5
> swap_avail_lists[3]: /* node 3's available swap device list */
> zram3   -> zram0   -> zram1   -> zram2
> prio:1     prio:2     prio:3     prio:4
> swap_avail_lists[4-7]: /* node 4,5,6,7's available swap device list */
> zram0   -> zram1   -> zram2   -> zram3
> prio:2     prio:3     prio:4     prio:5

By the way, when testing, I hacked zram kernel code to assign device_id
to zram->disk->node_id to emulate those disk with node_id. 

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 8acad3cc6e6e..b0bb6531b029 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -2719,6 +2719,7 @@ static int zram_add(void)
 	zram->disk->flags |= GENHD_FL_NO_PART;
 	zram->disk->fops = &zram_devops;
 	zram->disk->private_data = zram;
+	zram->disk->node_id = device_id % nr_node_ids;
 	snprintf(zram->disk->disk_name, 16, "zram%d", device_id);
 	atomic_set(&zram->pp_in_progress, 0);
 	zram_comp_params_reset(zram);