Re: [PATCH v3 6/7] mm, swap: remove swap slot cache

[Heiko Carstens <hca@linux.ibm.com>]

On Mon, Apr 28, 2025 at 11:31:59PM +0800, Kairui Song wrote:
> On Mon, Apr 28, 2025 at 9:53 PM Heiko Carstens <hca@linux.ibm.com> wrote:
> > > +     if (order) {
> > > +             /*
> > > +              * Should not even be attempting large allocations when huge
> > > +              * page swap is disabled. Warn and fail the allocation.
> > > +              */
> > > +             if (!IS_ENABLED(CONFIG_THP_SWAP) || size > SWAPFILE_CLUSTER) {
> > > +                     VM_WARN_ON_ONCE(1);
> > > +                     return entry;
> > > +             }
> > > +     }
> 
> The !CONFIG_THP_SWAP check existed before because slot cache should
> reject high order allocation. But slot cache is gone, so large
> allocation will directly go to the allocator.
> 
> It was not a meaningful WARN in the first place, and now the allocator
> should just fail silently for high order allocation, that's totally
> fine and expected and will just inform the caller to split the folio.
> 
> I'll just change the WARN_ON condition to `if (order && size >
> SWAPFILE_CLUSTER)` then, this should silence the WARN.

If I understand your suggestion correctly then this would be the
resulting code:

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 2eff8b51a945..5a7797143948 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1276,7 +1276,7 @@ int folio_alloc_swap(struct folio *folio, gfp_t gfp)
         * Should not even be attempting large allocations when huge
         * page swap is disabled. Warn and fail the allocation.
         */
-       if (order && (!IS_ENABLED(CONFIG_THP_SWAP) || size > SWAPFILE_CLUSTER)) {
+       if (order && size > SWAPFILE_CLUSTER) {
                VM_WARN_ON_ONCE(1);
                return -EINVAL;
        }

However, with that change I get this splat (and a few more) instead:

------------[ cut here ]------------
DEBUG_LOCKS_WARN_ON(l->owner != current)
WARNING: CPU: 4 PID: 934 at ./include/linux/local_lock_internal.h:52 folio_alloc_swap+0x22e/0x560
Modules linked in:
CPU: 4 UID: 0 PID: 934 Comm: swapon01 Not tainted 6.15.0-rc4-00021-gca91b9500108-dirty #2 PREEMPT 
Hardware name: IBM 3931 A01 704 (z/VM 7.4.0)
Krnl PSW : 0704f00180000000 000003ffe05231b2 (folio_alloc_swap+0x232/0x560)
           R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:3 PM:0 RI:0 EA:3
Krnl GPRS: 000003ff80000002 0000000000000003 0000000000000028 000003ffe2fe6c90
           0000000000000003 000003ff80000003 000003ffe19fc538 000003720322c000
           0000000000000000 000002f2ab38f000 000002f200000100 000003720322c000
           0000000000000000 000002f3f0eda278 000003ffe05231ae 0000037febbdb3f8
Krnl Code: 000003ffe05231a2: c020008e69bc        larl    %r2,000003ffe16f051a
           000003ffe05231a8: c0e5ffe2f4dc        brasl   %r14,000003ffe0181b60
          #000003ffe05231ae: af000000            mc      0,0
          >000003ffe05231b2: a7f4ff92            brc     15,000003ffe05230d6
           000003ffe05231b6: a7080001            lhi     %r0,1
           000003ffe05231ba: a7a80001            lhi     %r10,1
           000003ffe05231be: a7980000            lhi     %r9,0
           000003ffe05231c2: a7f4ff04            brc     15,000003ffe0522fca
Call Trace:
 [<000003ffe05231b2>] folio_alloc_swap+0x232/0x560 
 [<000003ffe046d2d2>] shrink_folio_list+0xe02/0x12d0 
 [<000003ffe046edc8>] shrink_inactive_list+0x188/0x5a0 
 [<000003ffe046f7d4>] shrink_lruvec+0x104/0x400 
 [<000003ffe046fd0c>] shrink_node_memcgs+0x23c/0x2c0 
 [<000003ffe046fe3a>] shrink_node+0xaa/0x420 
 [<000003ffe0470500>] do_try_to_free_pages+0xd0/0x5c0 
 [<000003ffe0472144>] try_to_free_mem_cgroup_pages+0x144/0x290 
 [<000003ffe057b33a>] try_charge_memcg+0x1ca/0x420 
 [<000003ffe057b5ea>] charge_memcg+0x5a/0x1a0 
 [<000003ffe057d5d4>] __mem_cgroup_charge+0x44/0x160 
 [<000003ffe04bb6ac>] alloc_anon_folio+0x27c/0x5d0 
 [<000003ffe04bbbac>] do_anonymous_page+0x1ac/0x7b0 
 [<000003ffe04bdec2>] __handle_mm_fault+0x212/0x4c0 
 [<000003ffe04be20c>] handle_mm_fault+0x9c/0x230 
 [<000003ffe015422c>] do_exception+0x1dc/0x540 
 [<000003ffe125c560>] __do_pgm_check+0x130/0x220 
 [<000003ffe126e3ae>] pgm_check_handler+0x11e/0x170 
INFO: lockdep is turned off.
Last Breaking-Event-Address:
 [<000003ffe0181c96>] __warn_printk+0x136/0x140
irq event stamp: 4120751
hardirqs last  enabled at (4120751): [<000003ffe126d0ee>] _raw_spin_unlock_irq+0x3e/0x80
hardirqs last disabled at (4120750): [<000003ffe126cca8>] _raw_spin_lock_irq+0x98/0xd0
softirqs last  enabled at (4104986): [<000003ffe018c1fc>] handle_softirqs+0x2fc/0x550
softirqs last disabled at (4104969): [<000003ffe018b9c6>] __irq_exit_rcu+0x126/0x140
---[ end trace 0000000000000000 ]---