[RFC 06/10] Reclaim memory from blocked kernel stacks

[Sebastian Andrzej Siewior]

On 2026-08-28 15:17:47 [-0400], Steven Rostedt wrote:
> On Fri, 28 Aug 2026 15:13:13 -0400
> Steven Rostedt <rostedt at goodmis.org> wrote:
> 
> > If one worries about stack size, they may want to turn off
> > CONFIG_RANDOMIZE_KSTACK_OFFSET as I see some big numbers for do_syscall_64()
> 
> And the scheduler has some heavy stack usage here!
> 
>   # trace-cmd stack
> (stack tracer running)
>         Depth    Size   Location    (31 entries)
>         -----    ----   --------
>   0)     8120      48   __msecs_to_jiffies+0x9/0x30

I didn't expect get_user_pages_unlocked() getting that big:

(stack tracer running)
        Depth    Size   Location    (60 entries)
        -----    ----   --------
  0)     8176      48   __css_rstat_updated+0x9/0x150
  1)     8128       8   __cgroup_account_cputime+0x2f/0x50
  2)     8120      48   update_se+0x10c/0x1b0
  3)     8072      48   update_curr+0x31/0x180
  4)     8024      56   put_prev_entity+0x153/0x1f0
  5)     7968     136   pick_next_task_fair+0x684/0x8e0
  6)     7832     120   __schedule+0x1ca/0x1130
  7)     7712       8   preempt_schedule_irq+0x38/0x60
  8)     7704      72   irqentry_exit+0x148/0x6b0
  9)     7632     136   asm_sysvec_apic_timer_interrupt+0x1a/0x20
 10)     7496     240   HUF_compress1X_usingCTable_internal_bmi2+0x12d2/0x1c40
 11)     7256      80   HUF_compress4X_usingCTable_internal+0x192/0x1d0
 12)     7176      32   HUF_compressCTable_internal+0x61/0x90
 13)     7144     136   HUF_compress_internal+0x2d2/0x500
 14)     7008      56   HUF_compress4X_repeat+0x25/0x60
 15)     6952     176   ZSTD_compressLiterals+0x1ad/0x400
 16)     6776     224   ZSTD_entropyCompressSeqStore_internal+0xfb/0x320
 17)     6552     112   ZSTD_compressBlock_internal+0x11b/0x210
 18)     6440     296   ZSTD_compressContinue_internal+0x232/0xdd0
 19)     6144      56   ZSTD_compressEnd_public+0x27/0x190
 20)     6088     112   ZSTD_compressStream2+0x9fc/0xa60
 21)     5976      80   ZSTD_compress2+0x83/0xd0
 22)     5896      24   zstd_compress+0x3d/0x70 [zram]
 23)     5872      72   zcomp_compress+0x62/0x90 [zram]
 24)     5800     144   zram_submit_bio+0x558/0xb00 [zram]
 25)     5656     128   __submit_bio+0x164/0x240
 26)     5528      96   submit_bio_noacct_nocheck+0x134/0x390
 27)     5432      64   bio_await+0xa7/0xb0
 28)     5368      16   submit_bio_wait+0x16/0x20
 29)     5352     184   swap_writepage_bdev_sync.isra.0+0x12e/0x230
 30)     5168      40   swap_writeout+0x18d/0x2e0
 31)     5128     176   shmem_writeout+0x286/0x530
 32)     4952     440   shrink_folio_list+0x588/0xfc0
 33)     4512     296   evict_folios+0x399/0xac0
 34)     4216     160   try_to_shrink_lruvec+0x1a4/0x390
 35)     4056      64   shrink_one+0xc0/0x1a0
 36)     3992     240   shrink_node+0xa9a/0xcf0
 37)     3752      88   do_try_to_free_pages+0xb3/0x4d0
 38)     3664     176   try_to_free_pages+0xce/0x220
 39)     3488     264   __alloc_pages_slowpath.constprop.0+0x8cc/0x1240
 40)     3224      96   __alloc_frozen_pages_noprof+0x2f6/0x340
 41)     3128      64   alloc_pages_mpol+0xb6/0x190
 42)     3064      56   vma_alloc_folio_noprof+0x6e/0xd0
 43)     3008      80   do_anonymous_page+0x32e/0x8f0
 44)     2928     224   __handle_mm_fault+0xb31/0xf60
 45)     2704      64   handle_mm_fault+0xee/0x2f0
 46)     2640     152   __get_user_pages+0x1b5/0x1140
 47)     2488     104   get_user_pages_unlocked+0xf0/0x330
 48)     2384     128   hva_to_pfn+0x2ea/0x450 [kvm]
 49)     2256      64   __kvm_faultin_pfn+0x61/0x90 [kvm]
 50)     2192     120   kvm_mmu_faultin_pfn+0x2cf/0x6f0 [kvm]
 51)     2072      24   kvm_tdp_page_fault+0x94/0xf0 [kvm]
 52)     2048     120   kvm_mmu_do_page_fault+0x1d9/0x210 [kvm]
 53)     1928     200   kvm_mmu_page_fault+0x7e/0x7b0 [kvm]
 54)     1728      40   npf_interception+0xba/0x240 [kvm_amd]
 55)     1688     168   kvm_arch_vcpu_ioctl_run+0x931/0x1900 [kvm]
 56)     1520     200   kvm_vcpu_ioctl+0x2e4/0xa10 [kvm]
 57)     1320      56   __x64_sys_ioctl+0x97/0xe0
 58)     1264    1072   do_syscall_64+0xe1/0x640
 59)      192     192   entry_SYSCALL_64_after_hwframe+0x76/0x7e

Does this stack tracer distinguish between the kernel-stack and
IRQ-stack? I would expect asm_sysvec_apic_timer_interrupt() on the
IRQ-stack and not adding weight to the kernel stack.
Given how close this is to 8192, I don't think two stack pages will
happen soon.

Sebastian