[Deepin-Kernel-SIG] [linux 6.18.y] [Upstream] [PATCH v5 mm-new 0/2] mm/swapfile.c: select swap devices of default priority round robin

[opsiff]

Link: https://lore.kernel.org/all/[email protected]/T/#u

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

The adjustment for swap device with node_id intended to decrease the
pressure of lock contention for one swap device by taking different
swap device on different node. The adjustment was introduced in commit
a2468cc ("swap: choose swap device according to numa node").
However, the adjustment is a little coarse-grained. On the node, the swap
device sharing the node's id will always be selected firstly by node's CPUs
until exhausted, then next one. And on other nodes where no swap device
shares its node id, swap device with priority '-2' will be selected firstly
until exhausted, then next with priority '-3'.

This is the swapon output during the process high pressure vm-scability
test is being taken. It's clearly showing zram0 is heavily exploited until
exhausted.

===================================
[root@hp-dl385g10-03 ~]# swapon
NAME TYPE SIZE USED PRIO
/dev/zram0 partition 16G 15.7G -2
/dev/zram1 partition 16G 3.4G -3
/dev/zram2 partition 16G 3.4G -4
/dev/zram3 partition 16G 2.6G -5

The node based strategy on selecting swap device is much better then the
old way one by one selecting swap device. However it is still unreasonable
because swap devices are assumed to have similar accessing speed if no
priority is specified when swapon. It's unfair and doesn't make sense just
because one swap device is swapped on firstly, its priority will be higher
than the one swapped on later.

So in this patchset, change is made to select the swap device round robin
if default priority. In code, the plist array swap_avail_heads[nid] is replaced
with a plist swap_avail_head which reverts commit a2468cc. Meanwhile,
on top of the revert, further change is taken to make any device w/o
specified priority get the same default priority '-1'. Surely, swap device
with specified priority are always put foremost, this is not impacted. If
you care about their different accessing speed, then use 'swapon -p xx' to
deploy priority for your swap devices.

New behaviour:

swap_avail_list: /* one global available swap device list */
zram0 -> zram1 -> zram2 -> zram3
prio:1 prio:1 prio:1 prio:1

This is the swapon output during the process high pressure vm-scability
being taken, all is selected round robin:

[root@hp-dl385g10-03 linux]# swapon
NAME TYPE SIZE USED PRIO
/dev/zram0 partition 16G 12.6G -1
/dev/zram1 partition 16G 12.6G -1
/dev/zram2 partition 16G 12.6G -1
/dev/zram3 partition 16G 12.6G -1

With the change, we can see about 18% efficiency promotion as below:

vm-scability test:

Test with:
usemem --init-time -O -y -x -n 31 2G (4G memcg, zram as swap)
Before: After:
System time: 637.92 s 526.74 s (lower is better)
Sum Throughput: 3546.56 MB/s 4207.56 MB/s (higher is better)
Single process Throughput: 114.40 MB/s 135.72 MB/s (higher is better)
free latency: 10138455.99 us 6810119.01 us (low is better)

Changelog:

v4->v5:

• Rebase on the latest mm-new;
• Clean up the relics of swap_numa in Documentation/admin-guide/mm/index.rst.

v3->v4:

• Rebase on the latest mm-new;
• Add Chris's Suggested-by and Acked-by.

v2->v3:

• Split the v2 patch into two parts, one is reverting commit
  a2468cc, the 2nd is making change to set default priority as -1
  for all swap devices which makes swapping out select swap device round
  robin. This eases patch reviewing which is suggested by Chris, thanks.
• Fix a LKP reported issue I mistakenly added other debugging code into
  v2 patch. clean that up.

v1->v2:

• Remove Documentation/admin-guide/mm/swap_numa.rst;
• Add back mistakenly removed lockdep_assert_held() line;
• Remove the unneeded code comment in _enable_swap_info().
  Thanks a lot for careful reviewing from Chris, YoungJun and Kairui.

Baoquan He (2):
mm/swap: do not choose swap device according to numa node
mm/swap: select swap device with default priority round robin

Documentation/admin-guide/mm/index.rst | 1 -
Documentation/admin-guide/mm/swap_numa.rst | 78 ---------------
include/linux/swap.h | 11 +--
mm/swapfile.c | 106 ++++-----------------
4 files changed, 17 insertions(+), 179 deletions(-)
delete mode 100644 Documentation/admin-guide/mm/swap_numa.rst

--
2.41.0

Summary by Sourcery

Revert NUMA-based swap device selection and introduce round-robin allocation among swap devices with the default priority, simplifying swap availability tracking to a single global list.

New Features:

• Select swap devices with the default priority in a round-robin fashion to balance swap usage across devices.

Enhancements:

• Replace per-NUMA-node swap availability lists with a single global plist head for simpler and more uniform swap device selection.
• Define a single default swap priority constant and apply it to devices that do not specify an explicit priority, avoiding implicit ordering based on swapon time.

Documentation:

• Remove the swap_numa documentation and its index entry to reflect the deprecation of NUMA-aware swap device selection behavior.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

This PR reverts NUMA-aware per-node swap device selection and replaces it with a single global priority-ordered list, then standardizes the default swap priority to -1 so that multiple devices with default priority are selected round robin, while still honoring explicitly configured priorities.

Sequence diagram for global round-robin swap device selection

sequenceDiagram
    participant Process
    participant mm as mm_swapfile
    participant head as swap_avail_head
    participant si as swap_info_struct

    Process->>mm: swapon(specialfile, swap_flags)
    mm->>mm: alloc_swap_info()
    mm->>mm: setup_swap_info(si, prio)
    note over mm: prio = DEF_SWAP_PRIO unless SWAP_FLAG_PREFER
    mm->>mm: enable_swap_info(si, prio, ...)
    mm->>mm: add_to_avail_list(si, true)
    mm->>head: plist_add(si.avail_list, swap_avail_head)

    Process->>mm: folio_alloc_swap(...)
    mm->>mm: swap_alloc_slow(entry, order)
    mm->>head: plist_for_each_entry_safe(si, next, swap_avail_head, avail_list)
    mm->>head: plist_requeue(si.avail_list, swap_avail_head)
    mm->>mm: cluster_alloc_swap_entry(si, order, SWAP_HAS_CACHE)
    mm-->>Process: swap entry allocated round robin across default-priority devices

Loading

File-Level Changes

Change	Details	Files
Replace per-NUMA swap availability lists with a single global plist and remove NUMA-based swap device selection.	Remove swap_avail_heads array and related NUMA initialization logic, introduce a single swap_avail_head plist protected by swap_avail_lock. Change swap_info_struct to store a single avail_list node instead of a flexible array of per-node avail_lists, simplifying allocation and initialization of swap_info_struct. Update helpers that add/remove devices from the available list (add_to_avail_list, del_from_avail_list, __folio_throttle_swaprate, swap_alloc_slow, swap_sync_discard) to operate on the global list instead of selecting by current or folio NUMA node. Delete swap_node() logic and the per-node priority tweaking that favored devices whose block device node_id matched the NUMA node, effectively reverting commit a2468cc. Remove dynamic allocation and initialization of swap_avail_heads in swapfile_init and the corresponding early ENOMEM failure path in swapon.	mm/swapfile.c include/linux/swap.h
Standardize default swap priority to -1 for all devices and rely on plist ordering for round-robin allocation among equal-priority devices.	Introduce DEF_SWAP_PRIO macro set to -1 and use it when computing the default prio in swapon, instead of decrementing a global least_priority counter. Simplify setup_swap_info() to take the computed prio directly, drop least_priority-based auto-decrement and per-node special prio handling, and set list and avail_list priorities to the negated prio for plist ordering. Remove swapoff() logic that renormalized negative priorities (incrementing prio and adjusting plist prio for remaining devices), since priorities are now stable and non-decrementing. Ensure swap allocation paths (swap_alloc_slow, folio_alloc_swap via swap_avail_head) naturally perform round robin among equal-priority devices by requeueing the chosen device in the global plist. Keep behavior that devices with explicit positive priorities remain ordered ahead of default-priority devices, preserving admin-controlled performance tuning via swapon -p.	mm/swapfile.c
Drop NUMA-specific swap documentation that described the removed per-node behavior.	Remove Documentation/admin-guide/mm/swap_numa.rst which documented NUMA-aware swap device selection. Clean up the mm admin guide index by dropping the swap_numa entry, aligning documentation with the simplified global swap device selection policy.	Documentation/admin-guide/mm/index.rst Documentation/admin-guide/mm/swap_numa.rst

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[APPROVALNOTIFIER] This PR is NOT APPROVED

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[sourcery-ai]

Hey - I've left some high level feedback:

• The removal of per-node swap_avail_heads and swap_node() changes NUMA-aware behavior significantly; consider adding a brief code comment near swap_alloc_slow explaining that the allocation is now intentionally NUMA-unaware to avoid future confusion or accidental reintroduction.
• Since least_priority and the auto-adjustment of negative priorities on swapoff are gone, review mixed-priority scenarios to ensure the new DEF_SWAP_PRIO handling still matches intended ordering when devices with explicit priorities coexist with default-priority ones, and clarify that behavior in the priority assignment code.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The removal of per-node swap_avail_heads and swap_node() changes NUMA-aware behavior significantly; consider adding a brief code comment near swap_alloc_slow explaining that the allocation is now intentionally NUMA-unaware to avoid future confusion or accidental reintroduction.
- Since least_priority and the auto-adjustment of negative priorities on swapoff are gone, review mixed-priority scenarios to ensure the new DEF_SWAP_PRIO handling still matches intended ordering when devices with explicit priorities coexist with default-priority ones, and clarify that behavior in the priority assignment code.

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[Copilot]

Pull request overview

This PR updates the swap device selection policy to stop using NUMA-node-specific available lists and instead perform round-robin selection among swap devices that have the default (auto) priority, while keeping explicitly assigned priorities ordered ahead as before.

Changes:

• Replace per-NUMA-node swap_avail_heads[] tracking with a single global swap_avail_head list.
• Assign a uniform default swap priority (-1) so default-priority devices are selected round-robin rather than “first swapon wins”.
• Remove NUMA swap selection documentation and its index entry.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
mm/swapfile.c	Switches swap availability tracking to a single global plist and adjusts swap allocation/rotation accordingly.
include/linux/swap.h	Simplifies swap_info_struct by replacing the per-node avail_lists[] array with a single avail_list node.
Documentation/admin-guide/mm/swap_numa.rst	Removes documentation for the NUMA-based swap selection behavior.
Documentation/admin-guide/mm/index.rst	Drops the swap_numa entry from the MM documentation index.

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